How many games can you play simultaneously?

My best for normal chess is 10, when I did a simul for my chess club, and 4 boards for bughouse.

How many can you handle before not being able to play at what you consider to be your "decent" or "optimal" capacity?

For me, it's how long I play, not how many. I can play for 30 min at optimal capacity. After that, I'll drink caffeine or a sports drink.

I think the answer is different for OTB chess vs correspondence chess. Here they don't have the capability of hosting simuls (yet) in live chess, but of course you can start as many turn-based games as you want. Paul Keres is famous for writing that he had hundreds of correspondence games going at any one time - and that is what he credits for his vast opening knowledge.

My answer: in turn-based chess, I had ~130 games going at one time, including perhaps 10 against people within 100-200 pts of my rating. It was ok for a bit but I'm glad I'm down to ~15 games. In a "live" simul, I don't really know - is there really a difference between doing 25, 50, and 100 boards at the same time? Other than the length of the event.

I suppose it depends on who I'm playing; for example, playing against one person better than me would require most of my concentration.  The most i've done without losing an obvious amount of playing ability was three at once (lame).  In bughouse, i tried to play both boards but failed lol.