So 6MP, 10MP & 12MP.
All were beautifully reproduced and looked stunning.
Thinking back to images I have had published, I remember images from my first "serious" digital camera, an Olympus E-10 (4MP) being published full page (A4) in magazines. I remember a glossy magazine cover from 2/3 of a Fuji S2 Pro Camera (6MP). I also had a 48 sheet hoarding poster printed from a Minolta 7i (5MP)
These images also looked good. Every bit the equal of images I had published in my film days from 6x4.5 or 6x6 transparencies.
Magazine printing is usually a four-colour process (CMYK) which is achieved by converting a digital file into 4 separate screens which determine the amount of each colour sprayed onto the paper. Cyan, Magenta, Yellow and Black (K) make up all the other colours. This is usually done at between 150 & 200 DPI (Dots per inch) Digital files are normally sent for printing at 300 PPI (Pixels per inch) which is double the print density. This is an industry standard & has been for years.
A 6MP camera produces a file of approximately 18MB. In terms of size it is approximately 10 x 7" or 260 x 175mm at 300 PPI.
An A4 magazine page is 8.3 × 11.7" or 210 × 297mm. Not much bigger than the 6MP file which is double the necessary DPI anyway.
A 12MP Panasonic m4/3 file is 13.3 x 10" at 300 PPI. An A3 Magazine Double page spread is 11.7 × 16.5" or 297 × 420mm. Again not much bigger than the file size.
So it can be seen that 6MP is OK for A4 and 12MP is OK for A3. In certain circumstances a 6MP file can work at A3, however it needs to be very sharp and interpolated carefully. Again its more about the quality of the pixels rather than the quantity. If you use Photoshop to increase the size of your files for reproduction, the software can only work with what is there. The better the original, the smoother the upsizing.
The advantage of larger files is not necessarily for reproduction, but to give the possibility of high quality cropping. Its very rare that I sell an image to a magazine, book publisher etc. and it isn't cropped. Designers tend to fit images to text rather than the other way around.
So more pixels is an advantage if you are intending to sell images for reproduction. But for the vast majority of use 12MP is enough to ensure high quality print resolution.