That number 30 was put out in the speculation days, it was not accurate, and it’s not part of the MP. You can imagine the difficulty of trying to establish a number. In a small town with a parish of 100, a group of 10 devotees might be significant enough to offer a “1962 Missal” Mass weekly or monthly.
I guess this is why the Holy Father used phrases conveying the idea of a stable interest, sincere interest, rather than a specific number.
How about people who are registered members, and have given gifts to the church? That’s what you had to have to vote in the vestry elections at my old piskie church. I thought it quite reasonable — note no quantity of gifts specified.