The figure usually given for Protestants worldwide is somewhere between eight hundred million and a billion. It is built on a residual definition: Western Christians who are not Roman Catholic. That definition sweeps in several groups who reject it.
Anglicans have argued about the word since the sixteenth century. A large part of the communion understands itself as a reformed catholic church rather than a Protestant one, and the Anglo-Catholic tradition rejects the label outright.
Anabaptists were persecuted by Lutheran and Reformed authorities as well as by Rome, and the historic peace churches descended from them have never regarded themselves as a branch of the magisterial Reformation.
Restorationists, the Churches of Christ and related movements, hold that they are not reforming a corrupted church but restoring the original one, and that Protestantism and Catholicism are two halves of the same wrong turn.
Latter-day Saints and Jehovah's Witnesses are filed as Protestant by some statistical agencies purely by residue, and neither accepts it.
Pentecostals and independent African and Asian churches, now a very large share of the total, often have no institutional connection to the Reformation at all and would not use the word about themselves.
There is a second problem underneath the first. Counting requires a unit, and Protestantism has no unit. There is no register, no communion boundary and no authority to be in or out of, so the number is assembled by adding up denominations that count differently: some count baptised members, some count communicants, some count adherents including children, and some report attendance. The totals are not comparable and the sum is not a measurement.
The useful statement is comparative rather than absolute. Whatever the total, most Protestants no longer live in Europe or North America, and the largest single-country Protestant populations are now in Africa and Asia.