It depends on what you mean by "how many datacrystal files will we need?" GT, and Chapter 3, can only take one datacrystal file at a time. You save out multiples from the previous game(s) and explore each if you'd like, but you'd have to swap to a new one for each possibility.
More broadly, what are the dc files for and what do they do? Primarily, they determine which of three additional scenarios you unlock in Haven as things progress: Boars, Hyenas, and Scratch. Boars and hyenas are unlocked by meeting the right triggers in the previous game. If/once you trigger their Haven scenario, that is ALSO tracked, as those scenarios have stages that unfold with each game (the second boar stage will be in Chapter 3's haven, but only if you triggered the first in GT). Scratch will be unlocked in 3 (eventually) if you manage to get through both CS and GT "Clean" (well, clean enough). The hyena and boar scenarios can exist side-by-side in 3 and onward, but you'll have to chose one or the other ultimately. Scratch's scenario is exclusive.
So let's consider:
If you wanted to see everything in GT, you'd need one DC file, with the requirements to unlock the boars.
If you wanted to see everything in Chapt. 3 (eventually, hyenas won't be in on release, nor will Scratch's scenario), you'd need 2 DC files: one with the boar scenario triggered in GT and the proper triggers from GT to start the hyena scenario in 3, and one that was "pure" so you could start the Scratch scenario. I suppose you could argue you technically need 4 if you want there to be a. a scenario with JUST hyenas in Haven b. A scenario with JUST boars c. A scenario with both d. A scenario with Scratch. But you don't get any extra content from being boar or hyena exclusive, it's just a neat little reflection of your choices up to that point.
So in practical terms, you'll need one DC file for Chapter 3 on launch to see everything, and a second variant eventually if you want to see the Scratch stuff.
And thanks! Will be fun to share how things are going so far.