Hi, I am an expat that runs a Spanish school and lives in downtown Querétaro, and would be happy to answer some of your questions. I don't know if there is a place to go to get the number of expats in the city, but the number can vary widely depending on what you consider and expat and what you consider the city.
I have heard the number 30,000 thrown around, but I have no way of backing it up. If by expat you mean native english speakers, the number would be about half of what it would be if you mean people of other nationalities.
And also, there are two distinct groups of expats in Querétaro (with some exceptions). Those who live here by choice, and those who live here because their work sent them down. The large majority of the people who are here for work live outside of the downtown area in the more modern suburbs (primarily Jurica and Juricia). As the previous poster mentioned, you can get in touch with this group through their club, the New Comers club. The webpage is:
INCQ Home
The other group are the retirees or business owners (like myself), who are in Querétaro by choice, or fate, or what-have-you, and live in the centro histórico. If you are interested in getting in touch with me through email, or some of the others who live downtown just send me a private message on the forum. There is currently no webpage for those who live in downtown.
If you have any specific questions, please do not hesitate to contact me.