You may not even need any new models, sounds, or maps. But for your first version, you probably won't need more than one person for each area of your mod (code, sound, models, maps).
Your first instinct might be to hire anyone who can code, or model, or make maps, and so on. When looking for team members, try to only hire people you absolutely cannot survive without. Counter-Strike's core team was one person. The more people you have, the more time spent managing them. Adding more people to the team doesn't mean more work will get done. If you're dealing with an on-line team, you can easily spend all your time managing it, and that means you won't be spending any time on making your mod.
Managing a team of people is a full-time job, even when all the team resides in the same building. The guiding rule here is to keep it small. Lets start by looking at how to assemble a team.
