Zombidle [quote] from user below: "I have collected 1D skulls and still can't upgrade carl to lvl11. is it a bug, or have I maxed its lvl?" [/quote]
You have to upgrade to level 9, then collect as many skulls you can, then buy MAX for carl to get him past level 10. You can only upgrade him once. So you can get from 9 to 12, or 9 to 15 etc but after that you can't continue to upgrade him. Hope it's clear.
The skulls needed for each level:
10 3.815N
11 99N
12 2.483D
13 62.088D
14 1.552U
15 38.805U
16 970U
17 2.33E+40
18 5.82E+041
19 1.455E+043
20 3.6375E+044
21 9.09375E+045
22 2.2734375E+047
The Great Greedoss 
Outdoor Basketball 
The Enemy is Near 