I have two characters. My first character is a level 20 with the main quest completed and a lot of side quests done. My second is around level 8ish. I haven't gone searching for Martin yet because I want to be able to explore without running into an Oblivion Gate. I was just wondering how many levels there were in this game, because I got every achievement in this game but I really want to get back into it. I'm unsure which character I should continue.

I don't think there's a level cap... I got my guy to level 70 and started all over because I finished every quest...

If you really want to hit the level cap, you'd have to go to jail in-game to get attributes lowered so that you can distribute points into skills (or stats like endurance, agility, etc?) which when fully maxed will prevent you from maxing out at 99.  Haven't played that game in awhile, so not too sure.

The reason I wanted to continue leveling up is because I basically rushed through the game trying to get those achievements.....I guess I'm just like that. I really want to get more out of this game because there are still a ton of locations I haven't discovered and I probably have a hundred dungeons and caves I should explore.

I think I only got to the high 20's by the time I have finished pretty much every quest and DLC the game had to offer by the end.

My dude is like level 30something, and I haven't even played Nights of the Nine or Shivering Isles yet.

No end-all level cap. But if you build your character and use the exploits as people are suggesting, you can reach something ridiculous.    
 
Ultimately, it's sort of pointless to go anywhere past the high twenties, especially after you've knocked out all the achievements.