Depends how much money you can lose without it affecting you. If you can lose heaps in one session and still play your A game then take 5-6. If you're likely to get tilted or annoyed when losing then capping yourself at 2-3 buy ins for the night could be a good idea.

Gday Sven, I think 2-3 buy ins is a good amount. Maybe just bring an even K so you can conveniently carry the good in one single chip.

Hey sven, I personally bring 4 buy ins with me when I go play, ive lost roughly 3 buy ins a few times but it is quite hard to hit huge downswings in soft live games, it has happened only once that I was in for 4 buy ins and called someone to bring me money because the game was too good! lolol...
If you are downswinging 5-10 buy ins frequently, its either time to start thinking about working on your game harder or try to evaluate better the games you are playing in (lineup and rake)