It's often one guy with a spreadsheet, and 1-2 people to review the spreadsheet and a few business people to validate assumptions.  The modeling is easy, getting the assumptions right is hard.                                
  To be honest, if you're skilled very simple models that you can do in your head or in a few minutes usually give you a perfectly good answer.  The more complex exhaustive models are usually there to make sure you didn't overlook something or 20 small inputs all cross multiplied to throw your answer off.                             
    Getting the answer exactly right also doesn't matter, if you make $90m in profit off a deal or $100m your going to do it.  What you are most concerned about is making sure that you don't lose money and what factors would push you to do that.