Here is a link to the pricing guide.
If I remove one user from 200 users,is it possible to replace the removed user with a new user and can I get the details of the removed user if I need?
Yes, each user account has a boolean for active. If you go to the account edit page, you will see a checkbox for this. If you uncheck it, it will deactivate the account and prevent any logins from it. Deactivated accounts will also not count towards your total number of users.
You even can do it even way smarter with your architecture, but this becomes consultancy :)...just pm me if needed