I personally sit for about 50xthe blinds. That is plenty to let you sit around for some hands. $1/$2NL that would add up to $150 ((1+2)*50). I don't think I've ever seen anyone else claim this, so I guess I can say I invented it
jk. Anywho, $150 at these stakes, if you folded every hand, and were 9 handed the whole time, you would see 450 preflop hands. Now, obviously, you're not going to fold 450 consecutive times. You will, however, have more than enough opportunities to play selectively enough that you will get strong hands to play with. That's just a personal number though. What you should do before deciding an amount is to look at what everyone at the table currently has. If the biggest stack at the table is $80, you wouldn't want to sit down for $400 (might be too tempting to bully, which could put you in some bad situations like raising with marginal hands to steal a pot and get called with a slightly better than marginal hand). Plus it would be pointless because the most you could profit in an all in situation wouldn't be your $400, it'd be that guy's $80. I'd say sit down anywhere near the big stack.