How can I update multiple drupal 7 sites on my server, using drush
You can simply create a Bash script, and feed in all the folder names in the sites/dir into Drush.

See this script for updating a Drupal site; modify it according your needs-

Remember that testing is a very important part of every site upgrade. You should include a manual testing phase after the pm-update, and then push each tested site back to the live site as they are found to be working.