As a buisness I don't think Square Enix will take a risk of making more than three parts. If you look at the amount of people who finish a game they buy, it probably isn't 100% of people who buy FF7R will finish FF7R. Then even less people will buy the sequel and even less the more sequels you do until they are not worth making anymore. If you look at Mass Effect, Mass Effect 2 was the most successful and then Batman Arkham City, also the second game, was also the most successul.
Even between FF13-2 and Lightning Returns there was a massive sales drop between the second and third game, although in the FF13 series the first game was the most successful. So I don't think they will take a risk of making more than three. They might do some spin offs and stuff after, but for the main story I think it will be three.
You know the thing I didn't agree with a lot of people about was how they said Midgar was 'only' the first 8 to 10 hours. Its 8 to 10 hours in length, but storywise there is immense story progression in Midgar. For example there are places in FF7 like the Mythril Cave and Mt Nibel. They take time to traverse, but hardly any story progression takes place in those dungeons. Midgar by comparasion is huge in terms of story progression. So hopefully the next one will be Kalm to City of the Ancients.
I would hesitate to draw conclusions based on those games. In both scenarios, the second entry was significantly better than the first (in most people's opinions), and the third game was a step down. There is likely to be more drop off the more parts you do, but I wonder if since they'll be reusing a lot of assets and such, the later entries may still be profitable even with declines in sales.