Controlling proliferation of postgres.exe processes

Greetings!

At the current moment, our customer's computer has 22 instances of postgres.exe running.  When a colleague checked a few minutes ago, there were 29.  Our contract specifies that we cannot consume more than 40% of the computer's memory, and we're over that level.  When does an instance of postgres.exe get created, and how can we make sure we create only the minimum number necessary?

Thanks very much!

RobR