level 50 is the soft cap. You can level after 50 but it takes much longer and you no longer get perks. Mathematically the game would cap you out around lvl 75 ish.

I'm currently level 20, 28 hours played.

Quests don't really have anything to do with levelling other than the skills you use to complete them.

There are no experience points - you just need to use different skills to level up. For example you could stand in one spot and summon a bound sword over and over for a week if you wanted and gain a bunch of levels.