Of the fifteen playing cards, one is currently a basic land, ten are common, three are uncommon, and one is rare or mythic rare. This distribution was introduced with Shards of Alara and facilitates Limited play (Draft and Sealed).
Booster Draft rules allow you to add as much basic land (only plains, island, swamp, mountain and forest) as you want to your deck and require that the deck be at least 40 cards. The standard number of lands in a draft deck is 17–18.
You, along with everyone else at the table, open one pack each and select—"draft"—one card from that pack. Then you pass the rest of the cards to the player on your left. The packs get passed around the table until all the cards are gone. You repeat this process for the second pack, passing to the right.
How many basic lands does a Cube need? 30 to 40 of each type is plenty for an 8-player draft. In a typical cube that supports 5 colors split among 8 drafters, it's unlikely more than 2-3 drafters end up in the same color.
Draft and Sealed For both formats, the standard number of lands to play in a 40-card deck is typically 17. However, aggressive decks may go as low as 15 lands if they're mono-colored, and 16 lands for 2-color decks.
How to draft: First, players sit around a table in a semi-circle. Each player then opens a booster pack and picks a single card without showing the other players. Each player then passes the remaining cards to the left, and continues drafting from the new cards they get from the player on their right.
This means that if your average mana value is three, which is fairly typical, then you should start with 25 or 26 lands and cut one land for every three or four cheap card draw or mana ramp spells in your deck.
Keep in mind that you're not allowed to look at your cards during the drafting process and after everyone is done drafting the first pack, there is a 90 second review period where you get to look at all the cards you've drafted.
How to draft: First, players sit around a table in a semi-circle. Each player then opens a booster pack and picks a single card without showing the other players. Each player then passes the remaining cards to the left, and continues drafting from the new cards they get from the player on their right.