You can just look at what we have left (roughly, there could be more islands but no combatants involved) but imo, theres a point where fights wont be traditional
- Egghead: Vs. Kizaru and his group
- Elbaf: Vs. Some isolated villain imo for now,
- 4th Poneglyph, Lodestar, LT: War between BBP, RHP, CG, and Marines so fights will be comprised of 3-4 people. Luffy will fight BB, Shanks and Akainu potentially together.
- Final Conflict: Imu and WG
So really 4, but the 3rd one is going to be super chaotic and a free for all. Thats how i see it, we cannot skip Elbaf and multiple groups will be after OP or stopping those that want it.