No single figure provides a consistent comparison across different religions because they all count members differently.
I have argued many times that number of congregations is the best statistic for comparison for number reasons. Some will argue that Mormons and Adventists have larger congregations than JWs. I have not found this to be the case and I have done extensive local research on this. In Scotland the average attendance in a JW congregation is around 70 and there are 116 congregations.
I have been to almost every Adventist congregation in Scotland and they average about 50. Glasgow is the largest congregation with around 80 or 90 on a typical pre pandemic Saturday. Then Edinburgh and Dundee have around 60 each. Paisley and Irvine are around 40 each. Faifley is a tiny congregation of less than 20. Crieff congregation has around 50. I have not been to the congregation in Aberdeen but I have heard that it’s not very big.
I have been to about ten Mormon congregations. (There are 36 Mormon congregation in Scotland) None of them had an attendance over 50 any time I have been, and some have attendances as low as 20. I went to a stake meeting in Paisley and counted 200 people in attendance. That represents 4 wards and 2 branches. (A branch is a small congregation - and still counted as a congregation in the congregation total.) That means an average of 33 people from each congregation. Which is in line with what I saw when visiting individual congregations.
So everything I have seen suggests that JW congregations are bigger than Mormon and Adventist congregations in Scotland.
The other advantages to counting congregations instead of competing measures of membership include: 1) the number of congregations can be independently verified whereas other figures could be fudged in various ways and 2) the number of congregations gives a good indication of trend in membership.
For example Mormons tend to count every baptised person as a member regardless of attendance. This means the membership number inevitably goes up no matter what. (They currently claim an incredible 24,000 members in Scotland) The number of congregations on the other hand has gone down from a peak of 42 to 36. And some of the remaining congregations have been downgraded from wards to branches. In other words Mormons have declined significantly in Scotland but you would have no idea from the membership figure, whereas the congregation number gives a better idea. Mormons also have churches in Glasgow and Edinburgh that are shared by more than one congregation, so JWs are not unique in that either.
JW congregations in Scotland have been steady at around 116 for decades. They are are lot more JWs than either Mormons or Adventists in Scotland, and they are maintaining their numbers better than Mormons.
To cut a long story short, I think that number of congregations is a very good indicator of the size and growth of religious groups because it’s a number that can be indecently verified and gives a good measure of the trend in the group’s overall size and committed membership.