255 is the cap for all stats until you reach level 60. From there, the caps increase by 5 per level:
In addition to these innate cap increases, you can purchase alternate abilities that raise your cap:
1: Increase Max Hitpoints by 680
2: Increase HP when cast by 680
4: Increase STR by 85
5: Increase DEX by 85
6: Stacking: Overwrite existing spell if slot 1 is effect 'STR' and < 85
7: Stacking: Overwrite existing spell if slot 1 is effect 'DEX' and < 85
8: Stacking: Block new spell if slot 1 is effect 'STR' and < 1085
9: Stacking: Block new spell if slot 1 is effect 'DEX' and < 1085
10: Increase Str Cap by 85
11: Increase Dex Cap by 85
1: Increase STA by 75
12: Increase Sta Cap by 40
After that, there are Serpent's Spine spells (note that the difference in numbers represent ranks of the spells):
1: Increase Max Hitpoints by 816/850/884
2: Increase HP when cast by 816/850/884
4: Increase STR by 91/93/95
5: Increase DEX by 91/93/95
6: Stacking: Overwrite existing spell if slot 1 is effect 'STR' and < 91/93/95
7: Stacking: Overwrite existing spell if slot 1 is effect 'DEX' and < 91/93/95
8: Stacking: Block new spell if slot 1 is effect 'STR' and < 1091/1093/1095
9: Stacking: Block new spell if slot 1 is effect 'DEX' and < 1091/1093/1095
10: Increase Str Cap by 91/93/95
11: Increase Dex Cap by 91/93/95
Note: Rank III may not exist in game.
1: Increase STA by 83/86/90
2: Increase Endurance Pool by 480/500/520
12: Increase Sta Cap by 50