15 to 30 would be the low and high numbers respectively. Can't calculate the average with knowing the specific number in each year and the number of years.
Seminaries lose over half their students every year? That's astounding. What does the SSPX do? Take in any warm body, then kick out the psychos?
In my opinion that's an encouraging statistic. It means they are discriminating. From what I understand, most US seminaries only discriminate against orthodox candidates and they will ordain just about anybody else.
They're similar to the Marines, "We Need a Few Good Men".
The SSPX priests are occupying the void left by the Jesuits many years ago.