Most Traditional masses worldwide are very well attended. The problem is that many people don’t even know they exist. More than a few Bishops flatly refused to allow them in their Dioceses and in some cases would not even allow the Mass to be celebrated in the same place more than once or more than once a month for that matter. Los Angeles is a prime example of that. Also it is not uncommon for those coming into the Church to be told by RCIA teams that these old masses should be avoided as they are not spiritually beneficial due to the lack of participation on the part of the laity and those that attend them tend to be judgemental and elitist.
Where they are permitted without excessive restrictions they have thrived. I point to two outsatnding examples that I am personally aware of. San Diego and New Orleans. Both of these cities have well established and very well attended Traditional Masses and have had them for years and years. In San Diego, the Mass was celebrated for a number of years in the Chapel of a mauseleum and the crowds overflowed in the surrounding areas of the entombed. Kind of spooky in a way but it drove home the point that all are part of the Church both living and dead. However, the Bishop in San Diego has now assigned the Traditional Mass it’s own parish and the Mass is now offered seven days a week on multiple occasions. In New Orleans, the Mass is celebrated weekly at St Patricks Church and on first Fridays also at St Patricks. These places are living proof that the Traditional Mass is alive and well despite what those who hate and despise it will tell you.
The congregations for these two places are in the mid to high hundreds I would estimate.