What do you mean “legit”? I.e. how many commercial services provide Video On Demand content? Tricky question as there is a lot of blurring at the margins. For example, all major TV channels have their own VOD service, but we would normally see them as primarily TV services. Likewise, technically any site which offers videos users can watch is VOD.
If you’re after a more narrow question about SVOD platforms then it intuitively feels easier to answer but I struggled to find any reliable numbers. The largest scale effort to track these across borders is by the European Audiovisual Observatory researching VOD in the EU. https://www.obs.coe.int/en/web/observatoire/industry/home-video-and-vod
Does anyone else have suggestions of where to look?