This is a wordpress core feature on most (all?) list type admin screen, you just go to the “screen options” (right below the admin bar) and change the “number of items per page” setting. This setting is stored per user per screen therefor each user can customize it to whatever works best for him.
IMO this is all you need, and there is no need to do any coding, but if you want to have different default value than 20, you should investigate where the value is saved in the user meta and set it upon plugin activation.