>My question was as follows is 12mpix enough on FX to print >what A4 picture or bigger?
A4? That's your question? Absolutely! I print gorgeously detailed 12mp originals at 24" x 36" and so do thousands of other photographers. A4 is only 8.25" x 11.25" so go print whatever you want from a cropped 12mp original. If you know your final printed output is going to be large, shoot tight rather than loose so you don't loose print size later due to excessive cropping of all sorts of junk that shouldn't have been in the composition in the first place.
For most intents and purposes, good composition solves a lot of problems and makes a lot of considerations about megapixels completely irrelevant.