Roughly one RevOps head per $21M of ARR — the ratio holds between $21M and $29M per head from $25M ARR all the way through $500M+. It breaks in exactly one place: below $25M ARR it compresses to about $12M per head, because the first RevOps hire is indivisible.
This is the most useful planning number available, because it is a ratio rather than a percentage and therefore survives the denominator problem. Sense-check it against your own selling headcount: the same source implies about 4.3 sellers per RevOps head for well-funded companies.
Every claim above comes from work LeanScale published. These are the sources.