It depends on the type and scope of the contract. Federal Acquisition Regulations do a fair job at keeping oversight and "fairness" (depending on a lot of factors), but at a minimum, you'd have:

* A large body of workers and analysts constructing the requirements * A few contracts officers working to codify those requirements into a request for proposal * A handful of technical contracts officers evaluating the mass of responses * A large pool of technical contracts officers and contracts officers ensuring that the statutory grounds of the proposals are met (verifying that yes, this is a small / veteran owned / minority owned business or yes, this business does have prior qualifications, etc.)

After you've separated the wheat from the chaff, and eliminated the obviously incapable parties, the team contracts down to 1 or 2 contracts officers and their staff. This team evaluates the technical feasibility against the requirements, asks a lot of questions to their own technical teams, and then ultimately, votes on the winner.