I have personally used a 50 buy in rule. I am a bit more risk averse, and I also play 8-10 tables. So to be sure I am not playing with scared money, I like to have 50 buy ins. Whether you are using 20 or 50 buy ins, as you move up stakes, my genreal advice is to move up slowly. What I mean is if you play 4 tables and you are ready to move up, play 2 tables at the higher level, and 2 at your current level. Then when you are ready, make all 4 tables the higher buy in.