What is the max level cap?

A tweet of the game director confirmed the level cap it's 40.

(content of tweet)

Level cap is 40

As mentioned by @user199176

You can get skill points from tombs and other secrets on the map. Explore and do stuff and you'll get more points.

UPDATE!

• The hidden ones DLC increased the level cap to 45.
• The Curse of the Pharoahs DLC pushes it even further to 55