You will quickly realize that you won't be able to fill the entire ability tree in Assassins Creed: Origins by the time you reach the level cap of level 40, but there are actually sources to getting more ability points than just the 40 you get from leveling up.
Firstly, once you hit level cap, you can still gain more experience points. You keep leveling up, but your level will not increase. Instead you get an ability point and stay at the same level. This means that you can keep killing enemies, doing missions and just playing the game, and you will keep getting more ability points as you do this. It's definitely possible to max out the ability tree if you do all missions, discover all locations etc.
Another way to get extra ability points is to go to tombs. Each tomb gives you 1 Ability Point for completing it, which adds up to a lot of totals points if you do all the tombs in the entire game.Hermit locations and War Elephants also give 1 Ability Point each, and combined with all the Tombs, you get a total of 23 for all these secret locations.