The book authors Barrett, Kurian, Johnson collected the data from many different sources.
We Catholics consider most non-Catholics generally as “Protestants” except for what the book calls - “Marginal Christians”. These other groups include such Johnny Come Latelys like: Jehovah’s Witnesses, Mormons, various “Arian” or pseudo-Christian cults, some Christian science or “mind science” cults, some Unitarian/Universalist groups, and tiny numbers of so-called Christian or Catholic “Gnostics.” There are about 16,000 of these.
To tie this into the OP let me say that since core Protestantism boasts doctrinal unity, if not ecclesiastical unity and equivalence, around the concept of “protest” and the 5 solas then it is only fair to lump them all together by their own standard. Hence any “ecclesiastical community” that uses the traditional Protestant Bibles (e.g. KJV) are related through (what Catholics consider a grave theological error) Sola Scriptura. So these can all rightly be considered together as just different variants of that same core cultish “protest” principal.
To wit: We have about 9,000 core Protestant sects if we don’t consider the independents. This includes the following sects:
Adventist ,Baptist, Christian Brethren (Plymouth Brethren, Open only) , Congregational, Congregationalist Disciple, Restorationist, Restorationist Baptist, Christian Dunker (Tunker), Dipper, German Baptist, Brethren Exclusive Brethren (Plymouth Brethren, Closed, Strict) ,Anglican Evangelical, Independent Evangelical Fundamentalist , Holiness (Conservative Methodist, Wesleyan, Free Methodist) , Lutheran / Reformed united church or joint mission, Lutheran, Mennonite, Anabaptist (Left Wing or Radical Reformation) , Methodist (mainline Methodist, United Methodist) ,Moravian (Continental Pietist) , Nondenominational (no church or anti-church groups) , Oneness-Pentecostal or Unitarian-Pentecostal: Jesus Only , Baptistic-Pentecostal or Keswick-Pentecostal , Holiness-Pentecostal: 3-crisis-experience
Apostolic, or Pentecostal Apostolic (living apostles) , Pentecostal (Protestant; Classical Pentecostal) , Friends (Quaker), Reformed, Presbyterian , Salvationist (Salvation Army) , United church (union of bodies of different traditions) , Waldensian community church or union congregation,
The largest of these out of 318 million total Protestants (year 1995 numbers for members, year 2000 numbers for denominations) are the Lutherans (61 million members, 253 denominations), next are Baptistic-Pentecostal/Keswick (49 million members, 396 denominations), followed by Baptists (48 million members, 322 denominations), Reformed/Presbyterian (44 million members, 300 denominations), Methodists (23 million members, 123 denominations), United church (22 million members, 54 denominations), Lutheran/Reformed united (15 million members, 24 denominations), and various Adventist groups (11 million members, 218 denominations). From these are formed nearly 9000 Protestant Christian denominations. Someone might complain about the “Oneness” groups being included since they reject the Holy Trinity (one God in three distinct Persons) and the historic Creeds, but that’s how Barrett’s Encyclopedia categorizes them, for whatever reason.
**If you count the “mega-bloc” of “Protestants” only it is 9000 / 33000 or 27% of the total. However, if you combine Protestants with Independents and Anglicans ( [22000 + 9000 + 168] / 33000) it is 94% of the total or 31,000+ . **
The bottom line is that the only thing each of these protestant sects really have in common is their KJVs (or other similar protestant bibles), baptism (most with water), “protest” against Catholicism, and factionalism. None will claim equivalence with the other and each think they are the One True Church.
**None of these various Protestant secs have a legitimate and traceable apostolic succession; none of them have valid priestly orders; and none of them have any divine given authority to exist outside of a human interpretation of the Bible handed down to them from the Catholic Church. All these Protestant sects were started by individual men conjuring up a new private interpretation of scripture and not one of them ever new a single original apostle of Christ or early Christian personally.

**
James