[G04]: "Each FRC team provides up to four Players (a Coach, two Drivers and an Inbounder)."
[G31]: "Only Inbounders may contact Basketballs; each Inbounder may hold a maximum of two Basketballs. During Teleop, Inbounders must remove Basketballs from the Corral immediately upon arrival. All Basketballs in the Alliance Station must be held by Inbounders once removed from the Corral."
[G33]: "Inbounders must remain behind the Starting Line during the Match."
G04 means there are 3 inbounders per alliance.
G33 says where they must be during the match. It appears that they'll have to throw through the Inbound Stations from behind the starting line (I expect this to be changed in a team update, but stick to the current rule for now).
Inbounders will also have to get balls as soon as they enter the corral. Because of the last sentence of G31, I don't think inbounders can throw the balls to each other (it would not be "held" while in the air), but could probably pass it. Another strategy would be having the inbounder who picks up the ball move towards the Inbound Stations and throw the ball into play.