That's my point. The most that you can give to any combination of projects is 100% total. If you have two projects, one of them will have to suffer, or you will be forced to give them both 50% focus.
I'm saying something different though. If you have 2 projects, you won't be able to split your time as effectively so you may have 40% focus for each, and so on. As you take on more and more projects, the time it takes to get back into the groove increases. You are likely to spend a lot of time moving between projects and this will disrupt your overall focus.
Ahhh now I see what you're saying. You make an excellent point.