Tesilid's regressions start when he comes of age iirc.
Anything before that [including Ailette saving him in the dungeon back when they were 10-11] is basically considered the "root lore" across all regressions.
While the regressions are like "branches" from those roots.
After Tesilid comes of age, Ailette then only exists on the 17th "branch" of regressions.
Meanwhile, Tesilid has to experience ALL of the "branches" of regressions, from 1st to 17th to god-knows-when.
sry for the tree comparison, but that's the simplest way of explaining how it works.

Im on chapter 63 and i cant seem to understand how the regressions work. Aliette says that when tutorial ends Tez will experience at least 100 regressions and on the 17th they will meet? Or that they are already con the 17th? Or is it that this is timeline 0 and when the tutorial ends Tez will experience his first regression and continue to do so until he meets Aliette again on the 17th where Tez will be killed at the start?
Sorry if i dont make any sense