Maximum skills = inborn skills + genetic potential + skills acquired from lessons. Those are influenced by black market items such as Aphrodite’s tears (the inborn skills are 50% more), water of youth/sisyphus stone (lesson gains are doubled) and pieces of cloud. There are a few calculators that can do the math for you such as this one ( https://www.howrse.tools/ ), otherwise you can do the calculations using the formula found in the book of howrse site.

On top of IBS + GP + lessons, you can also get up to 60 skills from foal games.

Thanks all of You!!