Ok, can’t give you info on every offshoot Baptist group, but as for the larger groups…from
Separated Brethren (Revised): A Review of Protestant, Anglican, Eastern Orthodox, & Other Religions in the United States, by William J. Whalen. ©2002,
Our Sunday Visitor we get figures for denominations in the US:
Southern Baptist Convention - 15,750,000
American Baptist Churches in the USA (formerly northern Baptists) - 1,500,000
National Baptist Convention of the USA, Inc. (mostly African-American) - 5,500,000
National Baptist Convention of American (offshoot of the other NBC) - 3,500,000
Free Will Baptists (anti-Calvinist) - 197,000
United Free Will Baptist Church (black version of above) - no figure cited
National Primitive Baptist Convention - 250,000
Primitive Baptist Churches (white version of above) - 72,000
General Baptists (Arminian) - 72,000
Seventh Day Baptists (date from 1672 in RI) - 5,200
Baptist General Conference (Swedish ancestry) - 134,000
North American Baptist Conference (German ancestry) - 44,000
He also mentions that there are Baptist denominations “serving members of French, Hungarian, Danish, Italian, Finnish, Norwegian, Polish, Romanian, Jewish, Mexican, and Czech ancestry.”
As I mentioned in another thread, I highly recommend this book for background on various denominations.