How many times can I have a sex in a month?
There's no solid number when it comes to how many times you can have sex.
Of course, just like with running or any other sort of exercise, there is a limit to what your body can physically handle before it becomes sore and tired. However, where that limit actually lies can be pretty different for different folks. If any of your bits and parts start to hurt, then that's a pretty good sign that you've reached your limit and should take it easy for a while.
Also, it certainly is possible to reach a point at which you simply don't want to have sex anymore for a while. Much of what happens in arousal occurs in the brain, and it is possible to become a bit tired of sex if you're doing it all the time or when you are not totally interested in having sex.
As long as the sex you are having is wanted (by all of the parties involved), isn't causing you physical pain and isn't interfering with the rest of your life, you can do it as much or as little as you wish. So rather than worrying about putting a number on how many times you can have sex in a given time period, why not just focus on only having sex only when you really want to and when it feels good?