The Alpha Delta Pi history has been digitized and is on the ADPi website,
www.alphadeltapi.org (links on the site go to the heritage/history sections). It's great reading (if you are a fraternity history buff). From the 1929 history:
"In the early days the membership was limited to twenty, but as years
passed the limit was removed and sometimes fifty or sixty names were
carried on the active rolls. When the society became national it was
found necessary to again fix a limit and
now no chapter numbers over
thirty actives."
The history also has copies of correspondence about the name change from Alpha Delta Phi to Alpha Delta Pi, with rather spirited exchanges between the ADPi president and W. Raymond Baird over the way ADPi was represented in Baird's Manual.
I wish other GLO histories were so conveniently available!