Let’s look a at 24 mpixels. They usually come as 4’000 x 6’000 pixels.
- If you print at 300 pixels/inch, 24 Mpixels will enable a 20 in wide print - iff you fill the frame.
- 4k has up to 4096 pixels horizontally, which means that a 24 Mpixel shot overfills a 4k screen.
24 Mpixels should be enough for any but the highest resolution films. And you can always stitch a few shots for better resolution (for better visibility of grain)
Higher pixel count allows for less tight shooting without compromising quality.
If you buy a lens, check the specifications. Many macro lenses cannot do 1:1 without additional equipment like bellows or extension rings.
Search the forum for a whole lot more information on technical details.