How to check how many times a user has logged in?
I don't think drupal keeps that data as standard. You might want to look at User Stats Module which does it for you.


  Provides commonly requested user statistics for themers, IP address tracking and Views integration. Statistics are:
  1. days registered;
  2. join date;
  3. days since last login;
  4. days since last post;
  5. post count;
  6. login count;
  7. user online/offline;
  8. IP address;