One, always. I commute by walking and public transportation so I always need to have at least one hand free to hold an umbrella, hold on to the bar on the bus, etc. If I have more than one bag on my shoulder, one of them is always falling off or swinging around to hit me (or someone else on the bus). I understand in principle carrying 2 to distribute weight better but for the life of me I cannot carry a bag on my left shoulder; it never stays put, so even if I have 2 bags I carry them both on my right shoulder so it's even more weight on that shoulder. Plus the more bags I have, the more likely I am to forget one of them on the bus or something.
This is why I only use big bags (fits my lunch and ipad and various other things like my umbrella and a cardigan). If I need to bring my laptop, I have it in a neoprene sleeve that then goes into one of my big bags. I have a big black leather tote and my Longchamps that will easily fit either of my laptops, and in a pinch I can get the smaller 13" laptop into most of my purses, although sometimes I can't zip them up.
ETA: I have an insulated zip-up lunch bag that I put inside my purse. I've learned my lesson after having multiple instances of leaking lunch containers (and it always seems to be curry that leaks!)
Like this, except mine zips shut on top. It folds up flat when it's empty:
http://www.amazon.com/Bring-Em.....s=home-gar
