It's hard to keep track of Independent, and to a lesser degree, Ecclesia Dei data, because the chapels shift, congregations split, Masses are moved, etc. There's no real "central authority" to keep the numbers. It's a bit difficult in the SSPX too, because people move around (for example, I'm on the roster as a parishioner in Mexico, MO, St. Louis, MO, (I think) and St. Mary's, KS. However, there is only one of me, not three.) Also, people convert and apostatize, babies are born, people go from the SSPX to the Indult, or Indult to SSPX, etc. It's very difficult, therefore, to get accurate information on this, and I don't think that what information is available is open for public perusal. What I do know, however, is that, at least in the SSPX, and probably all trad sites, the population is steadily increasing as the families have children, and the children grow up and have children of their own. By that factor alone, even though sadly a good number of them fall away from the faith when they get out into the world, tradition is "growing." However, that doesn't factor in conversions, and all that.
I think the best bet for getting this kind of information would be to approach every individual parish. I know that St. Mary's has anywhere from 2,200-3,000 souls right now, everybody I ask has a different figure. Mexico has, I believe, 176 on the chapel roster, and 90-100 who show up for Mass every Sunday. I don't know what St. Louis has, but I'd guess a couple of hundred. If you factor in the Indult in St. Louis, that'd probably be a few hundred more.
It begs the question, though, who counts as a trad? Are we measuring "average sunday attendance?" "Whoever could theoretically show up?" "Christmas and Easter Trads?" If you go by vocations, STAS is now full, and it wasn't a few years ago. If vocations are increasing, then that's probably a pretty good sign that Tradition is growing, spiritually, if nothing else.