EIA has the number of members of each CTA affiliate in the state. In his brief post there is a link to a PDF file with the affiliates ranked by number of members, as well as a spreadsheet file in case you want the information in some other format (alphabetical or by location, e.g.).
You might be surprised to learn how jealously this information is usually protected by CTA. In the pre-Janus days, information about (especially) agency fee payers was a very closely guarded secret.