If I were to hazard a guess, I'm going to say the remake will be at least 6 parts. One going from Kalm to Junon and the other going to either Nibelheim, one starting in Rocket Town, while also covering Wutai and Vincent's stuff if he and Yuffie aren't optional, and one to cover the northern continent and the period where Cloud's not around, and the final part covering putting from Cloud's brain back together to the end. At least that that's what I think based on the pace they went at. And honestly, I feel like they could in theory not take as long to complete the other parts as the first took.
For me to keep buying them I feel like they either need to streamline it into 3 installments, which I'm fine with if that means less padding. Or if they want to do 6-7 parts then they better not all be full price, especially when there would be middle parts where there's no big endings and if they keep tacking on horrible final boss fights to make up for that I'll be disappointed. If it's fast little chapters like Telltale games do where they're fine without a big final boss and sell it for 20$ or a subscription then I'm in.
There is no chance they do 5 installments, also Nomura is not the best game developer when it comes to following timelines. He does great work, but he appears to enjoy taking his sweet time when it comes to planning and development