It seems like I usually have at least 3 windows open while actively using, but I also depends on how close I am to power.

If I can plug it in, probably around 6. The problem is the phone - great once it gets going, but coming in from the background is not always a smooth experience. Missed a call yesterday because it was unresponsive while opening I believe I had web, email, maybe one other window open. I also run Conversations, Battery, Foreca, FM trans, shutter, timer and Media player widgets (in addition to a bunch of shortcuts)