My research shows that there are approximately 4,000 to 6,000 resume writers in the U.S. But some of those are career counselors who write resumes only occasionally (or who "consult" on resume writing, but don't write them themselves). Some of those are secretarial and typing services that "do" resumes, but don't necessarily focus on the craft of writing as heavily.
Many people trying to get a handle on the industry just look at the number of resume writers who are listed under "Resume Writing" in the Yellow Pages (SIC Code 73380101 = "Resume Writing Service"), but this is under-reported, according to my analysis.
Current SIC Code counts are between 800-1300 resume firms -- but my own database shows more bona-fide resume writers than this. (My database is compiled from these Yellow Pages listings plus the major professional associations, plus "self-reporting" resume writers -- that is, they don't belong to associations and don't list themselves in the Yellow Pages, but still write resumes. Some of these might be those secretarial and typing services that also offer resume writing -- or even career coaches/life coaches/therapists who write resumes but don't advertise that service publicly. Others are "newbies" who haven't yet launched their businesses and therefore don't advertise.)
Also, there is no measurement of what a "bona-fide" resume writer actually is. Do they write 1 resume a week, or 10? Are only certified resume writers counted (not the way I count them!). Do they have to actually WRITE resumes, or can they provide feedback to clients to help them write their own documents (I count both).
One analysis of a commercial mailing list targeting Resume Writing Services found 1012 listings.
1-4 employees -- 777 businesses
5-9 employees -- 141 businesses
10+ employees -- 94 businesses
I didn't find their sales data to be believable, so I won't share that information.
Businesses that are a single entity -- 921 (91%)
Businesses that are a headquarters for multiple branches -- 64
Years in business:
Under 1 Year - 10
1 Year -- 23
2 Years -- 25
3-5 Years -- 231
6-10 Years -- 200
10+ Years -- 523
Because this list was likely compiled from Yellow Pages listings, keep in mind that not all bona-fide resume writers in an area advertise in the Yellow Pages -- and that more established businesses are more likely to advertise than new resume writers.