It has been several years since I tried to put any numbers together, and now I don’t have the time to try to do so. Since the numbers were from several years ago, they may be a minimum; or they may not be much different; without research, it is pointless to speculate.
However, when I counted, the Archdioceses of Chicago seemed to have good information as to where the EF was being said; at the time there was well over three hundred parishes and missions, and something like 3.6% had the EF.
But that did not take into account how many masses were said in all of the parishes and out of all the Masses on a weekend, how many were in the EF. It was not clear that the 12 parishes were strictly EF; there simply was no information. So that is not to say that 3.6% of Masses were in the EF; only that many parishes had at least one EF weekly/
From what I have been able to gather, there are some EF parishes within the US; however, it appears that the much greater tendency is that a parish will offer one EF, and the rest of the Masses (1, 2, 3 or more depending on the size of the parish and the number of priests available) are of the OF. This would seem to indicate that significantly less than 1% of all Masses offered on a weekend are the EF. And the EF may have 450 people attending (a number I saw recently in reference to one EF Mass); and the parish itself may have twice that attending the OF; or another parish may have 2000 attending. There are also the parishes that may have 150 attending the OF on any given weekend.
So it is nigh impossible to say, of those attending Mass weekly, how many attend an EF weekly.
The bottom line is that while there is tremendous enthusiasm among those who attend the EF, they appear to be in a significantly minor part of those attending regularly.
As to the statistics in other countries, I don’t know if anyone has any adequate statistics. The short of it is that the Ordinary Form is just that - the Ordinary form.
“The use of the old Missal presupposes a certain degree of liturgical formation and some knowledge of the Latin language; neither of these is found very often. Already from these concrete presuppositions, it is clearly seen that the new Missal will certainly remain the ordinary Form of the Roman Rite, not only on account of the juridical norms, but also because of the actual situation of the communities of the faithful.” This quote is directly from the letter by Benedict 16 accompanying SP.
A further quote: “Needless to say, in order to experience full communion, the priests of the communities adhering to the former usage cannot, as a matter of principle, exclude celebrating according to the new books. The total exclusion of the new rite would not in fact be consistent with the recognition of its value and holiness.”
As to the future, at least as Benedict saw it: “For that matter, the two Forms of the usage of the Roman Rite can be mutually enriching: new Saints and some of the new Prefaces can and should be inserted in the old Missal. The “Ecclesia Dei” Commission, in contact with various bodies devoted to the usus antiquior, will study the practical possibilities in this regard.”
If anyone in any position of authority on the matter has had anything to say, Pope Benedict did, and it seems that is not something that at least some who favor the EF want to hear. However, it is likely that there will be some changes to both the EF and the OF (as the most recent changes to the OF should be a good indication).
What seems to be missed is that Vatican 2 did not start this whole process; it was well under way during the reign of Pope Pius 12th if not before. People look at the documents of V 2 and the Mass promulgated after, and see none of what went before. The move to modify the EF started long before Vatican 2. Had it been a gradual change over time rather than such an abrupt change, some of these divisions might not exist. But the likelihood of going back to the EF alone would appear to be non-existent. The greater likelihood is that there will be minor changes to the OF and the EF over a longer period of time, and some of those changes may only be matters that are within it currently. But whether the form is EF or OF, it is clear the mind of the Church was to move to vernacular; to what extent Latin may be required only in the OF (as opposed to being an option) is subject to speculation. But the far greater majority of laity, priests, bishops and cardinals support the vernacular, including the past 4 Popes and the current one.