Short answer lots.
More and more programs are now opening the research tracks. I think to attract research oriented applicants. There are some programs that have mandatory research year, like UCSD.
When you apply on ERAS it lists the regular and the research track and you can apply to both or just one. Or if you are not applying this year, you can browse through program websites and see what the offer. But it takes time.
I am not sure how many people are interested. The general vibe I got is people would not want to do it unles they have to. Meaning they will apply to those tracks and will do them, if they won't match into a regular 5 year track. But besides that, not very many are applying/ranking the 6 year tracks as their top choices. But again there are some.