How Big Must a Group Be for the Latin Mass?

The Summorum Pontificum blog has a link to a fascinating article posted on the blog Catholic Church Conservation. A portion of the post is cited here:
A prerequisite for the request for the old Mass is the existence of a "fixed group of believers." The German Bishops are attempting to use this point against the Motu proprio 'Summorum Pontificum'.

The latest edition of the German theological journal 'Una Voce Correspondence' has published an article with the title "A Canonical Note on " Summorum Pontificum "and its practical implementation".

The author is the Right Revd. Father Wolfgang Rothe, Canon Lawyer and former Subregens of the Seminary of St. Pölten.

The paper deals with the question of how big a group of believers must be to request the old Mass, or whether such a group needs to be of a particular size.

Father Rothe also raises the problem, as to what is to be understood by the phrase "enduring existance" of this group.
Please check out this fascinating discussion.