Anabaptism is a theological family rather than a single organized denomination; its living members already appear in the membership records of its daughter bodies, so no source totals them as one population without risking double counting.
What would resolve this A scholarly survey that explicitly sums the reported membership of every living Anabaptist-descended body on a stated date, correcting for any overlap between them, would give the family a real combined figure comparable to a single denomination's.
OpenF. L. Cross and E. A. Livingstone (eds.), The Oxford Dictionary of the Christian Church, 3rd revised edition