The number is an estimation, of course, made by Christian Marquant for the French liturgical association "Paix liturgique".
This number is reached with the following subgroups.
(1) Around 760 priests who belong to the Society of Saint Pius X (SSPX) and religious communities friendly to it.(2) Around 600 priests from those communities previously called "Ecclesia Dei" (that is, those established regularly following the motu proprio Ecclesia Dei).(3) Around 130 priests from religious communities that were never under the authority of the now-defunct "Pontifical Commission Ecclesia Dei" (for instance, the priests belonging to Fontgombault and its daughter-houses, or Norcia, etc).(4) At least 3,000 diocesan priests (maybe up to 5,000, if one includes those who are very cautious because of their local bishops) who celebrate it regularly, even if not exclusively, including 1,000 at least in the United States: this is the Summorum Pontificum group, by far the largest.
Marquant adds: "Our 4,500 priests attached to the Traditional Mass (1500 "Trad" priests to which are added 3,000 diocesan or regular priests) represent at least 1.1% of the global Catholic clergy (or more, if we consider only the Latin Church priests, since it is a Latin liturgy, and if we would consider only active priests) who have remained or become Tridentine, which is far from being a ridiculous number if we consider that this identity was forbidden for a long time and remains widely persecuted. And, despite this, it is growing..."
(Paix Liturgique, via Le Salon Beige)
