For FSSP, ICKSP, SSPX, CMRI, RCI, IMBC, SGG
What are the populations of parishioners (including those who live far away and watch online masses from them) for each of these groups?
And what are the numbers of priests/brothers/monks/nuns for each of these groups?
I would find this information interesting as well. Too bad such info has NOT been collated, ever.
There is no Nielson or Pew Research for Traddieland. There's no central organization like Rome or the Pope who has power over the world of Tradition. Google "Crisis in the Church", subheading "Vatican II". Ideally all the Trad groups would be mere Religious Orders under the same Catholic Church authority/structure, and there would be no Crisis in the Church. The Church would take its own census, and release its own official numbers. But we have a Crisis in the Church, so here we are.
Just for starters, you're talking about independent organizations which probably don't give all their "business" customer data out to the world at large. Even if they reported # of parishioners on their respective websites, it would be a BEST GUESS on their part, and not any kind of hard, scientific, or accurate figure like the US Census. In the worst-case, it would be an exaggeration or FOND WISH about their numbers, to make themselves look more like winners. Everyone wants to be part of the winning team. Human Psychology 101.
We're talking about lifeboats. Many Trads go to independent chapels affiliated with no larger group which might have a big website. That necessarily leaves out a crucial piece of information out of any "TOTAL NUMBER OF TRADS" figure. Also, Trads mistrust the government, are above-average concerned about privacy -- both of which are good things to-be-expected -- so I doubt a very high percent of Trads would consent to fill out any substantial survey.
Heaven only knows how many Trads attend a Mass associated with no "group" larger than 5 priests. How many Trads live almost completely offline, not only shunning Social Media but even all websites and forums? There are plenty.
Some Trads don't even want to be included in "parish directories" for large established groups like the SSPX. And thinking back to my SSPX days, I remember that parish directory being hopelessly out of date -- about 2 years after it was printed. People who had moved, passed on to the next life, gave up the Faith, went to the Indult, people I had never seen (myself being a regular and active parishioner), etc.
And even if a chapel released its "mailing list" or parishioner list -- how many of those are current, active members? As in, how many of them are likely to attend the very next Sunday Mass? I have personal experience that demonstrates that the size of one's mailing list is NOT the same as who shows up for every Mass!
So it's especially hard to get a snapshot, with so much in flux.
But when you consider all the variables, especially the nature of Trads and the distributed, guerilla, off-grid, resistance-movement nature of the Trad movement, it's not just near impossible but ACTUALLY impossible to ever get a 100% solid picture. You'd have to comb the entire country, looking for "priest holes" like during the English persecutions.
Recall that chapels can hide just about anywhere: rented facilities, homes, trailers, garages, basements, warehouses, you name it. Many groups are word-of-mouth or e-mail list only. So you can't find the chapel, you can't find the group.