I don't know how many....I use foils on the water and land. But I also use the occasional Slingshot inflatable and have a few (and rep them) and my partner uses them extensively, as do our customers.
I think the numbers you see are skewed. Not many ride foils on the water. they are primarily used for land do to their ease of setup and crash-resistance characteristics, and they sink so beginners tend to learn on inflatos and stick with them. However, since most known injuries and incidences involve inflatables doesn't mean anything when you think of the huge disparity between the two groups. Probably 19 of 20 riders use inflatables as opposed to foils.
Sure, as the sport progresses, bridle-less foils may take over, but you have to look at the overall facts. More inflatables out there means more injuries relating to them.