How many account we will create and what grant given to each account in mongodb administration and management ?

Question

We are using php, mongodb v 2.6.10, we have not set connection pool. We want to set connection pool at server level.
Presently we are creating a new connections every time, when application talk to database. When our no of connection reach to 400, our db performance impact so badly, and query time increase up to 4 times.