Wait, there's a new version of FFIV coming out?! GIMME GIMME GIMME
I think that the iOS version of TAY was the last one they made. But between FF4 and its sequel I bet there are 20 versions out there.
According to Final Fantasy Wiki, there are about 9 distinct versions of FF4. It is available on 11 different platforms. Final Fantasy 4 The After Years is available on 5 different platforms and there are 4 distinct versions of the game. The Interlude is only available with Complete Collection versions, meaning that you can only play it on 2 platforms, but it's the same version.
So that's about 13 versions of the game and its sequels.