CleverNickName
Limit Break Dancing
To answer the original question in the thread title: D&D has a level cap at 20 because levels need to cap somewhere, and 20 is as good of a number as any. There is nothing stopping you from capping all class levels at Xth level, where X is any number between 1 and 20.
This is really easy to dial in yourself. Let's say you've decided that you want the level cap to be 6th level. (I'm sure you have your reasons.) So characters would just stop leveling up when they reach 14,000 xp and would earn ASIs when they reach the proper amount of XP for them. Like this:
0 xp = 1st level
300 xp = 2nd level
900 xp = 3rd level
2,700 xp = 4th level, ASI
6,500 xp = 5th level
14,000 xp = 6th level (the cap)
34,000 xp = ASI
100,000 xp = ASI
195,000 xp = ASI
355,000 xp = ASI
This is really easy to dial in yourself. Let's say you've decided that you want the level cap to be 6th level. (I'm sure you have your reasons.) So characters would just stop leveling up when they reach 14,000 xp and would earn ASIs when they reach the proper amount of XP for them. Like this:
0 xp = 1st level
300 xp = 2nd level
900 xp = 3rd level
2,700 xp = 4th level, ASI
6,500 xp = 5th level
14,000 xp = 6th level (the cap)
34,000 xp = ASI
100,000 xp = ASI
195,000 xp = ASI
355,000 xp = ASI