Level 20 is the max level you can reach. Once you reach 20, you can go to the character-select screen and "Promote" your character. Doing this will reset your character to level 1 and gets rid of all your abilities, but also adds a permanent stat to all of your characters (+1 Constitution for promoting Melee, +1 Cunning for Rogues, +1 Willpower for Mages). Also, do not worry about losing your items. Required levels for item equipping is not a worry in multiplayer!