Re: How Many At Once
Well, there are a lot of variables to work out here.
First off: What's the posting rate in each of the games you're joining?
If you're expected to post once per day, you'll have less time for more games than, say, a game where you're expected to post once per week or even "as you like it." Coupled with this is how detailed your posts are expected to be. If you have to post 2-5 paragraphs per day per game, that's going to be a little rough if you get a few games going.
Secondly: What sort of free time do you have?
IF you're unemployed, or if you can play while at work, then you've got a lot more time to fill than someone with full-time employment as, say, an RN or even a CIA Agent. How much free time you have to dedicate to the games is going to either vastly limit, or greatly increase the amount of games you might handle to get into that "sweet spot" you're looking for.
Thirdly: Are you a GM or a Player in these games?
Obviously, GMing requires a lot more time and effort than being a player, and so how many games you're in as a GM will have greater weight than if you're a player. A game where you're the GM is (for me) weighted about twice as much as a game where I'm a player. Other GMs may weight it more like 3-4x, but that's a mileage may vary thing. Of course, all of this is. ;)
Last, but not least, some general advice.
I'd start off with 3-4 games, none of which require/expect multiple posts per day. If you can swing it, get one or two that require daily posts, and one or two with more lackadaisical posting rules. Figure out which you like more, or if you like the mix pretty well. Adjust from there.
No matter how many games I have, or other people have, everyone's different. ;) Me, I can easily handle 5-6 games at a time (depending on posting rates), but that's because I have way too much free time and I'm also a writer, so it's what I do anyways. Now, ask me if I'm IN 5-6 games at once, and I'll tell you that I'm not. I'm a little too picky for my own good there.