How many of each land do I need to support a draft tournament?
In a booster draft, generally a player has about 16-18 lands depending upon their card choices. So for a 10 person draft, you can assume around 160 - 180 lands in use at one time. However, because of the random nature of the cards in the pack you can't assume the distributions of the colors for each player, so you should probably assume that you need at a minimum 180/5 = 36 lands of each type. I wouldn't feel comfortable holding a draft unless I had 50 or so of each color, though.

The next question you need to ask is if you are allowing the participants to keep their land or if they need to give it back. MTG players are kinda forgetful in giving back basic lands, so you should probably consider having as many lands as possible so that you don't run out as you hold multiple tournaments.