Hi Sreenu,
It depends on your database use case requirements.
Assuming you are installing the current version of MongoDB v3.2, and using the default storage engine WiredTiger see Memory diagnostics for WiredTiger Storage Engine, especially How do I calculate how much RAM I need for my application?
You may also find the following resources useful:
I would also recommend to enrol in a free online course at MongoDB University to learn more about MongoDB. Check out M102: MongoDB for DBAs to learn more on how to administer a MongoDB installation.
Best regards,
Wan.