On the Turbo, there are four (if you count Gunhed/Blazing Lazers). I do, because I believe they all belong together, in spirit. Three if you don't:
(Gunhed)
Super Star Soldier
Final Soldier
Soldier Blade
There is one on the NES:
Star Soldier
There is one on the Nintendo 64:
Star Soldier: Vanishing Earth
There may be others on other systems but those are the only ones for those three consoles, and the only ones that I own.
Why did they make so many? I don't know, why did they make so f*cking many Final Fantasy games?
EDIT: Black_Tiger posted his reply while I was typing mine. Since I already took the time to type it all out I'm posting it anyway rather than hit "Delete." And yeah, he's right, they are all different and distinctive games with pretty unique identities.