A 24MP APS-C dslr to digitize a whole frame of 120 isn’t enough resolution? How much resolution do you need? 4000x4000 or 4000x5000 will make a pretty nice 16x16 or 16x20 inch print. Need to print bigger than that? You should probably go to a bigger negative as 16x20 is a pretty big enlargement from what is essentially a 2x2.5 inch negative. If you step up to a 4x5 neg, just use a flatbed scanner.
The whole stitching a bunch of photos of a frame of film is totally overrated, as it is extremely time consuming, prone to less than great results, and at the end of the day isn’t generally as good as just taking one picture of the whole film frame with a reasonably modern sensor digital camera. If you really want to hit it with resolution, buy a used Canon 5DSr and take 50MP photos of each film frame.
I think you’ll soon discover that 24MP in one shot is plenty of res for pretty much anything within reason. Trying to scan and stitch multiple frames isn’t really going to give you more usable resolution. For 35mm, 6000 pixels to scan 36mm of film is ~80 line pairs per mm of film.
While I agree that photographing frames single shot with a high resolution camera is the preferred option (when your budget allows this), in most cases even a 50Mp camera is not sufficient. I think your numbers are a bit optimistic. Here are some examples.
Example 1 - 35mm film, 16Mp camera, single shot.
Camera: Pentax K-5II
Sensor: 4928x3264 pixels (3:2)
Film format: 24x36 mm (3:2)
1" = 25.4 mm
Horizontal 36 mm, that is 36/25.4 = 1.417"
Resolution: 4928/1.417 = 3470 dpi
Example 2 - 35mm film, 24Mp camera, single shot.
Camera: Pentax K3-II
Sensor: 6016x4000 pixels (3:2)
Film format: 24x36 mm (3:2)
1" = 25.4 mm
Horizontal 36 mm, that is 36/25.4 = 1.417"
Resolution: 6016/1.417 = 4240 dpi
Example 3 - 35mm film, 16Mp camera, stitched in 4 parts.
Camera: Pentax K-5II
Sensor: 4928x3264 pixels
Film format: 24x36 mm
Stitch: 2x horizontal and 2x vertical + overlap 3 mm
1" = 25.4 mm
Sub frame horizontal 36/2 + 3 mm = 21mm, that is 21/25.4 = 0.826"
Resolution: 4928/0.826 = 5960 dpi
Example 4 - 35mm film, 50Mp camera, single shot.
Camera: Canon 5DsR
Sensor: 8688x5792 pixels (3:2)
Film format: 24x36 mm (3:2)
1" = 25.4 mm
Horizontal 36 mm, that is 36/25.4 = 1.417"
Resolution: 8688/1.417 = 6130 dpi
Example 5 - 120 film 6x4.5, 24Mp camera, single shot.
Camera: Pentax K3-II
Sensor: 6016x4000 pixels
Film format: 56x41.5 mm (Pentax 645N)
1" = 25.4 mm
Position frame horizontal, fill out vertical in sensor, 41.5/25.4 = 1.63"
Resolution: 4000/1.63 = 2450 dpi
Example 6 - 120 film 6x4.5, 50Mp camera, single shot.
Camera: Canon 5DsR
Sensor: 8688x5792 pixels
Film format: 56x41.5 mm (Pentax 645N)
1" = 25.4 mm
Position frame horizontal, fill out vertical in sensor: 41.5/25.4 = 1.63"
Resolution 5792/1.63 = 3550 dpi
Example 7 - 120 film 6x4.5, 50Mp camera, stitched in 2 parts.
Camera: Canon 5DsR
Sensor: 8688x5792 pixels
Film format: 56x41.5 mm (Pentax 645N)
Stitch: 2x vertical + overlap 5 mm
1" = 25.4 mm
Sub frame vertical 56/2 + 5 = 33mm, that is 33/25.4 = 1.30"
Resolution: 5792/1.30 = 4450 dpi
You can read ppi or lpi instead of dpi.
Some conclusions:
1. A 16Mp camera is not sufficient to scan a 35mm film (24x36) single shot (Example 1).
2. You can scan 35mm film with a 24Mp camera with more than 4000 dpi (example 2).
3. When you are stitching even a 16Mp camera for scanning 35 mm film can give plenty of resolution (example 3).
4. A 24Mp camera is not sufficient to scan 120 film (6x4.5) single shot (Example 5).
5. A 50Mp camera is not sufficient to scan 120 film (6x4.5) single shot (Example 6).
6. With a 50Mp camera you can scan + 2-part stitch a 120 film (6x4.5) with 4400 dpi (Example 7). This number is reduced for 6x6 and 6x9.
So even with a 50Mp camera you can not get enough resolution in most cases and the only solution is to stitch.
On average you need at least 4000 dpi for a good scan, but it depends what you are doing with it. For a large print you need more resolution. When scans are downsized and only shown on a monitor even scanning a 35mm film with a 16Mp camera single shot can be sufficient.
Edit 25/04/19: Example 5 corrected.