There are 70 total Abstergo challenges, so completing half of them will get you this achievement. You will still need to get 65 or more to get other achievements.
To see them all, press
to go to the pause menu. Then select Abstergo Challenges and press
They are grouped into 7 categories.
Go through them all to check their progress.
and Assassin's Creed Rogue Remastered Walkthrough
are good places to use for assistance with the more difficult challenges. The link goes right to the page you need.