For playing NL10 cash games ($0.05/$0.10), I usually try to keep around $300–500 in my bankroll.
One full buy-in for me is $10, so I believe it’s better to have at least 30–50 buy-ins.
For me, it looks like this:
minimum — $300
comfortable — $500
very safe — $700+
If my bankroll is only $100–150, one bad day can hurt my balance a lot, so I prefer to always have a good reserve.