There are two hardcoded limits. One is 20 for each stat maximum. Second is racial modifier, that raises or lowers that limit. Backgrounds also affect reachable maximum, but only for negative values (if background raises a stat, you won't be able to exceed the 20 limit, but if background lowers a stat, amount will be deducted from the limit).

Modifying byte at offset 0xB0FF8 to value 0xEB removes racial modifiers and all races will use default limit values. List of these values starts at offset 0x1B5204.