For me, it's how many characters I can juggle, and settings I can adapt to.
Although I don't have a set number.
I feel the same way. I usually try not to do too much at a time, and usually base how much I can handle on the specific Roleplays themselves as well as how many characters I can juggle.
Nothing's more awkward then responding to a post with the wrong character/account, lol.
Also, like Oniya mentioned, it sometimes depends on the speed of my partners as well. If we're going post-for-post back-to-back, then I tend to devote more time and attention on that one RP, however, if all the RPs are ticking by pretty slowly, say, a post a day, I tend to spread myself out more.
Unfortunately RL likes to call at random hours of the night, so my consistency is very... dynamic. I also try to take that into consideration and will usually only do one or two RPs at a time until I am certain I can devote more time to extras.