There is no "magic number" of which I'm aware, however, it must "represent a stable population" before permission will be granted.  In 2010 in the Diocese of Wilmington, DE, 85 adults of a Maryland parish petitioned the Bishop for an "extraordinary" Mass to be said on Sundays.  They were turned down on the basis of only one of the signers, a "lay deacon," living within a five mile radius of the church.  (It was presumed few would travel this far to hear Mass.)  What was NOT taken into account was that the church was located along a tidal canal in the middle of an industrial park!  The persons living within five miles of the church were the resident pastor, two nuns, and the deacon and his family--all on church property!  Everyone else in the parish lived beyond a five mile radius of the church!  The bishop simply did not want a Latin Mass accessible to residents of the "Eastern Shore."  The closest was either St. Athanasius in VA, Fr. Ringrose >150 mi. one way, or the 'motu' in Baltimore, also about 2 1/2 hours away.  One is entirely at the mercy of the personal whim of the Concillar Bishops.  What constitutes a "stable population"?  Whatever the bishop says!  Classic Modernist word-play!