how many times a collection angency can call in one day

They can call anytime from 8am to 9pm as many times as they want unfortunately

If you ask them to stop, they should. Do you know if their purpose of calling you is legit? Have they validated the account on which they are collecting from you? If you haven't received anything in the mail, write a DV letter and mention to communicate only through email, no phone calls. They must stop calling you or else the FDCPA laws will be violated.

In the last post you mention to communicate only through mail, no phone calls.Can I tell them this when they call on the phone? Or is this something that must be done through the mail? Thank you.

You can tell them anything you want. If you want to make it happen, however, send a letter using a delivery verification process (certified, UPS, etc...). If they continue to call after receiving the cease verbal communications, file a complaint with the FTC, your state's AG (and the CA's state, too), and/or get an attorney.

Also, to answer the first question, it can also depend on the state you reside in. Many states have laws protecting consumers from abusive collection tactics. Some are less restrictive than the fdcpa (and hence irrelevant), and some are more restrictive than federal law. You should be able to check with your state's consumer protection agency to get the applicable number of attempts/contacts.