Hi Ben,
I currently work at a smallish university in the Midwest, and we have approximately 170 faculty members with just four IDs. (I consider this an untenable number, but I'm not in charge of these things.)
Previously, I worked at a large online university, where there were over 1,000 faculty members (mostly adjunct), and there was an entire course development department that included about 100 members--with the tasks covered by most instructional designers split into different roles (i.e., IDs, developmental editors, project managers, course producers (technical experts), multimedia designers, and managers). Of that roughly 100 members, I'd say the IDs were probably 40-45%. That's a really rough estimate. Oh! And there were always at least 10 ID contractors. So, let's say 55 IDs in total.
I hope that helps!