100 buy-in is ideal for you to be able to 'overcome' the variance that you'll surely pass, but I think it depends a lot on how many tables you'll play...
For example. if you're only playing just ONE table cash game $0.01/$0.02 and you consider yourself a good player, I don't see any problem with you having only 20/30 buy-ins.
If you choose to play with 20/30 buy-ins, set a 'loss limit', as they do in the stock market. In the stock market, they always try to sell a stock as fast as possible to avoid larger losses and this is how you should treat your bankroll.
Set a loss limit such as 3 buy-ins in a row, for example.
Lost 3 buy-ins in a row? Stop playing and go relax for a while, returning to the game the next day.