Canon doesn't know that itself and it probably gets the information from the same place as anyone else: CIPA (if not Gartner, etc.)
I don't know if a random person can buy more detailed reports from CIPA or they limit the reports to the manufacturers. Point being Canon won't know how many cameras Sony, Nikon, etc, sell and have to get that information from an external entity such as CIPA. This means that Canon itself doesn't know the size of the market and can only guess based on its unit sales.
The 45MP sensor is for a unit that is aimed at producing 4k & 8K video. The R5 sensor is 8192 x 5464, 8k video is 7,680 x 4,320. 7680/8192 is 0.9375. The 5DS sensor is 8688 x 5792. 7680/8688 is not a vert nice fraction (and is around 89%). I expect that the R5 sensor size has therefore been chosen by Canon because of what it can deliver for 8K video rather than what it can do for stills photography. The 5DS wasn't targeted at 8K video, it was targeted at high resolution photography. The 5Ds's problem was that it was based on old generation sensor technology and produced low quality images for the resolution.
This isn't the best way to put it because there isn't an infinite selection of camera models to choose from with varying capabilities. I can't, for example, choose a R5 that only does 1080p or an R5 that only does 4k, my choice is an R5 (II) that can do 8k or not an R5. If I want 45MP in a Canon camera I have to buy one that does 8K video. Canon is turning the R5 into the "it does everything" camera. What this is doing is pushing the price up because Canon needs to expend more effort to produce a camera that can do everything well. $300 is a big price hike for air vents.
If Canon came out with a 60MP (or more) R5s that only did 1080p or 4k video, had no air vents, and sold for the same price as the R5 or less, it would be interesting to see what sold best - the R5II or R5s.
8K is effectively a dead video format as far as consumers are concerned (going the way of 3D.)