While I believe that there is no actually declared limit and the number of mail recipients of php mail() function is theoretically unlimited, I also believe that, from your experience, there is some maximum number that should be not be passed over in order to send php mail that will not end up in junk folder.
What is that number?
My task is to send approx 2000 important notification emails to organisation’s partners.
Most web hosting companies will limit you to about 100 (per hour I believe). Check with your host.