There's not much doubt that the original number was three, and that the song was originally English. Several writers of WWII memoirs mention singing the song in the early '40s. Ed Cray ("The Erotic Muse," 2nd ed., p. 121) quotes a (19th century?) morris verse   collected by Alfred Williams, that seems to have inspired it:


Oh, dear, what can the matter be?
Three old women tied to an apple tree!
One ran away, the others stopped till Saturday.
Oh dear, what can the matter be?


From Nat Green's column "The Crossroads," in the American show-biz paper "The Billboard" (Dec. 4, 1943), p. 37, datelined "Chicago":


"They'll find heady corn at Old Heidelberg rathskeller, where Louie convulses 'em with his burly bullfight and Eleven Old Ladies Locked in the Lavatory."


That would seem to beat Brand's version by by several years.

But if it's the world's record you seek, try Australian Don Laycock's "The Best Bawdry"(p. 182), with a cast of no fewer than twenty-one old ladies.

(Laycock admits to having enhanced some of the songs he prints.)

BTW, what was your mum's version?