http://yugioh.wikia.com/wiki/Exodia
The short answer is 3 times.
The long answer is extremely long:
If you scroll down to "Exodia in the anime series", about halfway down the page, there is a list of every duel in which any body part of Exodia (head, left and right arms and legs) has been played or used.
There is the time Yugi beat Kaiba for the first time by summoning Exodia. Those pieces were later lost in the sea, thrown in by Weevil Underwood, although Joey recovered two pieces.
One of the Rare Hunters beat Joey in a duel using Exodia. In Yugi's "rematch" against that Rare Hunter, the hunter uses the same tactic (but Joey is forced to say nothing to Yugi about tactics) but Yugi uses chain destruction to move all Exodia pieces to the graveyeard.
In the Virtual World, Gozabura (Kaiba's dad) uses an Exodia deck but tries to summon Exodia Necross not Exodia.
Exodia is used one final time (in Yu-Gi-Oh) in episode 216, but from what I can tell (that series hasn't been shown in the UK) it was defeated, even though it was summoned.
That would make 3 summonings, not including any in the real world (obviously) or any spin-offs (Yu-Gi-Oh GX, Yu-Gi-Oh 5Ds, etc.).
P.S. Thanks for the question, I only recalled Exodia pieces appearing in 3 episodes.
