How much beer can be lost if it is poured incorrectly resulting in too much foam?

It would depend on whether or not the foam overflows the receiving vessel. As long as the foam does not go over the lip of the glass, it (the foam) will slowly dissipate, transforming into liquid beer. If it overflows the vessel, then it would be that amount which is actually "lost" since once it turns into liquid, it will not be recoverable. Potentially, you can lose the entire beer to an overflow of foam, but it would require some effort.