It's unknown. Space Marine foundings aren't necessarily grand occasions per se-- it's possible for one Chapter to be founded in a given Founding, and that be the end of it. The Exorcists come to mind-- they were commissioned more or less as an experimental unit to see if exposure and subsequent exorcism made a formerly possessed individual less susceptible to a new possession. Obviously, they are closely watched by the Inquisition. The point remains, though-- it's possible for a single Chapter to be founded. It's also possible for dozens to be founded, as was the case with the 2nd Founding following the Horus Heresy. Part of why there's no real way to know how many Space Marine Chapters are out there is because there's a lot of missing data regarding the assorted foundings, namely about dates and numbers of Chapters activated.