If your bb/100 drops which it will if you add more tables, you will have bigger downswings and longer breakeven stretches. However, the fact that you are playing so many more hands means that these stretches will actually be shorter in terms of time and the more hands you play, the closer you will be to your actual winrate so it kind of cancels itself out. If you just experiment and play as many tables as you are comfortable with then you'll probably be close to maximising your $/hr which is what it's all about really.