It's all about appropriate start-points and end-points IMO.

Based on that, I think it will go like this:

Part 2 (FFVII Rebirth) - Kalm/Nibelheim flashback to Nibelheim/Mt Nibel (So starts and ends with Nibelheim)

Part 3 (FFVII Resurrection) - Rocket Town, Tiny bronco sea exploration, Wutai (compulsory), Gold Saucer Date, Temple Of The Ancients, Bone Village, Sleeping Forest, City Of The Ancients

Part 4 (FFVII Reunion) - Northern Continent, Icicle Inn etc, all the way to endgame (So starts and ends at the northern continent)

It really is hard to say though. It really depends on how much extra stuff they add. If they add as much extra stuff to everything else as they did to Midgar, then by that logic it would be about 10 parts lol. They have A LOT to cover, and I personally don't know why so many people think it will be 3 parts, it surely has to be more