It's hard to say for sure. But Marvel comics have always had a wide fan base, so I think a significant number of people read the early ones.
Well, estimating the exact number is tricky. But given Marvel's popularity over the years, it's likely that a considerable number of comic enthusiasts were drawn to the early issues. The appeal of those early stories and characters likely attracted a diverse readership.
I don't have an exact figure. But considering the influence and longevity of Marvel, it's safe to assume that a substantial number of people were interested in and read the early comic books. Maybe it was a few thousand or even more. Who knows for sure?