ujm,
on one hand, I looked up the literature again, and see indeed that the data is unclear. Some say it is tensof thousands
on the other hand, I don’t think it is counter-intuitive that those communities were started by small groups of traders. do we have records of large community migrations? Northern Europe experienced rapid growth due to advanced in agriculture (beans->proteins->kitniyos) and outgrew early centers and Ashkenazim benefited from that rapid growth.
What is your alternative data?