I'm doing some market analysis and there seems to be very limited information on how many SEO firms there are out there selling services.
The reason you're having problems finding data on the number of SEOs selling services is because:
1. Site designers offer SEO services, so the numbers will be mixed;
2. Anyone who's ever logged on can call themselves or their companies SEO/SEM professionals; and
3. SEO, as a means of growing a business, takes on less importance as connectivity between sites increases. A site visitor is more likely to find a site through a link than through a search of Google ergo, the number of SEOs keeps shrinking.
So, the reasons you're finding it difficult to locate data is because (1) SEOs are often designers, coders and other web denizens and (2) any idiot can call him/herself an SEO.
Oh, wait, I AM one of those idiots.
Good luck with the research,