totally up to you dude.
if you aren't necessarily favorites to win the tournament you may need more paint to make up the difference in skill level. In general, you can hide a lot of ugly underneath a team shooting a ton of paint. Say 3-4 cases per person.
But I have seen teams win full XBALL lite tournaments buying 1000 rounds each. I have seen 4-man paintball players use pump guns and go deep in the final rounds with all pump guns.
In general, at least have enough for 2 cases each, that is a safe number. If you find yourself in finals and want to buy more, then go for it. The playoff experience is definitely worth it