How many lashes did Christ receive, and what was the reason for that certain number?

Most sites don't refer to the number of lashes inflicted on Jesus. Some say the number is unknown. This site says Jesus was probably scourged 39 times.

https://web.archive.org/web/20020219102431/http://www.pixelworks.com.ph/shroud/scourging.htm

In 2 Corinthians 11:24, St. Paul speaks of receiving "forty lashes less one." Whipping a person 39 times was the standard practice in NT times.

Moses' Law refers to flogging. The law itself means forty lashes less one, or 39 lashes. The term was meant to be a biblical one in that 40 lashes are what was determined enough to kill a man according to the Old Testament and thus 39 lashes was the most you give a man without declaring a penalty of death. This was also the number of lashes Christ received from Pilate and thus it would have been un-Christian to flog someone more than that. In actuality 39 lashes was more than enough to cause a man to pass out and easily enough to kill. Quite often a captain or crew would mete out fewer lashes depending on the particular infraction. Moses' Law was usually deemed just for only the most serious crimes that did not carry a death sentence.

What is interesting, however, is there is no such Biblical Law referring to 40 lashes as a Sentence of Death! It was actually an old Roman law/tradition that said forty lashes were a death sentence. In Roman times, it was deemed that if a flogger were to appropriately administer a punishment, he should be able to kill a man with forty lashes. Depending on the circumstances, if he failed to kill a man in forty lashes, the flogger would face death. This was to ensure the flogger would not hold back in meting out the punishment. (Imagine being told to flog a friend or fellow slave and if you haven't killed him by the 40th lash you would be put to the sword!) Using this same warped twisted logic, the Romans determined 39 lashes shouldn't kill a person so that was the most you could give a person, without a sentence of death by flogging. Seeing how Jesus was to be crucified, the Romans were "merciful" and only gave him 39 lashes. Or perhaps the flogger was fearful of a death sentence if Jesus survived the fortieth lash.

https://lovepeacetruth.wordpress.com/2010/12/11/40-1-39-lashes/
https://web.archive.org/web/20040114095424/http://hegewisch.net/blindkat/pirates/punish.html

I have heard from my mother, who I suppose learned this at her Sunday School (Episcopalian) that he was sentenced to 40 lashes, but that it was against Roman law to give someone a stronger punishment than they were sentenced to, so as a matter of course, they were often given one less lash than sentenced in case they miscounted during the lashing.

This is a short answer from a software program I have called "Robertson's Word Pictures," which started life as an old print commentary. Copyright © Broadman Press 1932,33, Renewal 1960: "Thirty-nine lashes was the rule for fear of a miscount (Deu_25:1-3). Cf. Josephus (Ant. IV. 8, 1, 21)."

Adam Clarke's commentary in a much lengthier discussion bears out the same idea. Deuteronomy limits the number of lashes or "stripes" to forty, and the Israelites/Jews were punctilious to not accidentally exceed that number.

Flogging. Historians who spend their lives studying these things (unlike you or I who don't) say the 39 lashes came about because it was firmly held that 40+ would kill a man. So the 39 lashes were initiated to bring the average man to the edge of death.
https://web.archive.org/web/20060307223110/http://grault.net/adjunct/index.cgi?BrucePennington/TruthOfTheBible

Tradition permitted 39 lashes –- 40 to fulfill the law, less one to show mercy.
https://web.archive.org/web/20050204150820/http://www.weirdload.com/shroud.html

A scourging in Roman times was 39 lashes. A "cat of nine tails" was used, with each tail having a piece of metal or bone imbedded in the end of it. After the soldier struck Jesus Christ with the lash, the soldier dragged the lash across Jesus Christ's body in a whipping motion. The piece of metal or bone shredded Jesus Christ's flesh as the soldier whipped it across Jesus Christ's body. Thirty nine lashes was the legal limit, because 40 would kill a man. Because this was a "cat of nine tails" it was actually 39 times nine, or 351 lashes. Sometimes the prisoners were disemboweled by the beating. This was a society that had gladiators, men who fought to the death in the Coliseum for sport. This society would later turn hungry wild animals loose on Christians in the Coliseum. These cruel and hardened soldiers made sport of the flogging. The objective was to do the most damage without killing a man. Forty lashes would kill

I know this question is super old, then again, so is the topic thus it is timeless. Trying to not duplicate things said above, there are just a couple differentiating facts to consider to sort out this answer. First, Mosaic Law, introduced through Moses, stated 40 lashes to be the max, if the crime deserved such punishment at all. Less was certainly likely and at the discretion of the judge and completely based on severity of crime. No more than 40 was allowed, but the reason cited was because it would publicly humiliate and degrade the person, not because it would kill them, which in no way could the whips used do. This law was in effect starting ~1400 B.C.

Romans had something entirely different. They used a flagellum whip (similar to the British cat-o'-nine-tails) for their scourging punishment (called verberatio). This whip used ball-bearings and shards to first hit the surface with the ball causing an instant swell with the barb/shard following it, shredding the skin. Many times arteries were exposed and skin hanging. It was insanely cruel and degrading by design. It was intended to bring someone up to the brink of death (but often would kill the offender as there was no definitive # of lashes). They implemented the 40-1 simply as a notion it was unlikely anyone could survive 40+ -- period. But sometimes they used it as an outright method for a death sentence.

Two entirely different methods and purposes. The Romans, let alone the flagellum used, didn't even exist when the Mosaic Law was given. Additionally, crucifixion was another insanely cruel device of the Romans. So cruel were they, neither of them were legal for a Roman citizen to be sentenced to.

In Jesus' case, he was under Roman discipline, handed over by the Jews, so no Mosaic Law was administered. Unfortunately for Jesus, He got two different punishments combined. As far as we can tell, nobody was ever sentenced to both verberatio AND crucifixion. Pilate only did the scourging to appease the Jews who wanted to kill Jesus. He did not believe Jesus was guilty of anything. So he had Jesus scourged in attempt to appease them and then release Him. So He probably didn't even receive 39, or at least only if the lictor thought it would not kill Him, as that was not the purpose. Note Pilate the entire time did not think Jesus deserved any punishment.

However, when Pilate brought out Jesus after the scourging, but ridiculed as the centurions would often do (in this case, dressed like a King), the Jews blew up. To avoid a riot and realizing getting nowhere, Pilate very reluctantly agreed to also have Him crucified, saying the [unwarranted] blood of Jesus be on the their own heads (the Jews).

Hope that helps sort it out.

Sources:
Bible (Deuteronomy 25:1-3, John 18-19, Mark 15, Matthew 27)
Reams of history on Roman laws and administration