22762 rows in my session table. What does this mean for the site, and what should I do about it?
The long term action is to limit session lifetime.
Look for the following line in your settings.php:

ini_set('session.gc_maxlifetime',   200000);


This parameter sets session life time, after which session is marked as garbage and will be deleted by garbage collector.
Average session life time (not on drupal sites) is much less than 200000 seconds  - something between 15 minutes and half an hour (900-1800).
So, if you don't need long sessions, you can reduce ini_set value and thus get less sessions stored in the sessions table