I think the best you're going to get is probably Rifts
Steam Chart. As you can imagine, it's far from accurate. But you can compare it to other games on there to get an idea of the relative size of the population. There are other MMO's on Steam that have both their own launcher and use Steam too, but the best ones to look at in my opinion would be
Tera and
Wildstar, both of which use a similar model to Rift.
You could compare it to one with a known quantity of players like
FF 14 to get a very rough estimate of the percentage of the population that use Steam vs the games own launcher and use that to get an idea of how many people are playing Rift. I would in no way present these kinds of findings as being anything like accurate however, but it is one way to get an estimate.
As of posting this however, there are plenty of games that are still going through
Greenlight that have more regular Steam players than Rift. Even Trove has more! Rift is even performing worse on Steam than No Mans Sky. Rift may or may not be performing well through Glyph, but on it's not doing well at all on Steam.
Unfortunately the only people who really know the answer to this are Trion themselves, and they're keeping tight lipped on the subject.