The French organisation Paix liturgique estimates that between 4,500 and 6,500 Catholic priests worldwide celebrate the Old Latin Mass. This would be 1.1% of the Catholic clergy.
This number includes 760 priests of the Society of St Pius X (SSPX), 730 priests of other traditional communities like the Fraternity of St Peter (FSSPX) and others, as well as 3,000 to 5,000 diocesan or religious priests who celebrate the Tridentine Rite regularly.
The latter group received the permission to say the Old Mass in 2007 through Benedict XVI's Summorum Pontificum.
According to Vatican statistics there were 414.696 Catholic priests worldwide in 2016.
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The article fails to explain how this French organization reached such an estimate. I can say with certainty, the figure given for the FSSP's membership is factually incorrect.