How can we set the number of logins for awarding the points to the student?

You can set this restriction from WP Swings -> Points and Rewards for Tutor LMS -> Points on Login Frequency -> set the Max No of Login Frequency. In this condition, the points will be awarded after the threshold has been completed. 

E.g, Max number of Login Frequency is set to 5, and the number of points to be awarded is 50. So, when a student will log in and log out 5 times then only these points will be awarded.