Internal reps for large advertisers and agencies alone would exceed that number. Long tail advertisers can self-serve, but ad platforms need their whales and whales expect inside reps.
I posit brand ads require more staffing, because they require more sales efforts to get advertisers to buy. Direct response is easier to measure the ROAS, but for brand ads, it's a really good idea to have someone selling ads constantly to your existing clients. Otherwise, many clients in the mid-size space ($100k-$5M/year, approximately) probably can't measure their return, which means those budgets are always at risk of being cut.
With direct response, even data-naive decision makers can figure the value of.