I do not think you can see how many people are using Leap 15.0 looking at the number of downloads, because by far the majority of users will simply have used the online upgrade method (changing repos from 42.3 to 15.0 and doing zypper dup) and they do not download the ISO.
Downloading the ISO is more something done by new users.
Due to some technical issues in our infra, it will be very hard to come up with metrics that are more than a guess. From what I remember from the previous Leap releases. What we have seen here in the forums, is that the formerly usual peak in posts after a new release is ( almost ) absent. Same goes for the social media pages. And, from the positive reports I guess that a decrease in numbers is not what is going on, so ( and we as the forums team more or less concluded so ) it must be the quality and stability of Leap 15.
FWIW, the numbers would definitely have been 6 digit.
There’s also the point Henk mentions, although I know of quite some people who use the DVD iso for upgrading.