A 12MP file is not limited to 12.5 × 10 inches as a hard maximum. That size is simply what you get at about 300 dpi, which is a common target for close-viewed, high-quality photo prints.

You can print larger if you accept a lower print resolution. Larger prints are often viewed from farther away, so they can still look good even at lower dpi. The tradeoff is that if you stand too close, you may start to see pixelation or reduced fine detail.

A sharp image from a good lens and accurate focus helps the print look better, but it does not increase the number of pixels in the file. In other words, sharpness improves quality within the available resolution, but it doesn’t change the basic print-size math.

So: 12.5 × 10 inches at 300 dpi is a guideline, not a maximum. You can go bigger; the acceptable size depends mainly on viewing distance and how critical you are about visible detail.