As Cesar Alves said: level 21 is the goal and the score will be mixed from both single-player and coop matches.
I didn't find any exact numbers on how many points you need in total to get to lvl 21, so I will post my stats here for you to see how much longer you have to play.
Keep in mind that it does not matter if you finish with a particular rank. The ranks won't level you. The maximum score is what it is all about.
Here we go: Single player
1. not played
2. not played
4. not played
5. not played
7. not played
8. not played Coop
I did most of them on easy difficulty. Except for no. 3 in single-player.
Hope this helps you guys :-)
Any improvements? Please let me know in the comments below