Perhaps one way of regrouping the denominations is by lines of authority. If your church does not report to any higher authority (beside God), your are by definition your own denomination. It doesn’t matter if you are 1 church strong or hundreds or thousands. It also doesn’t matter how similar your beliefs are as long as you are not compelled to obey.
It is clearly stated that:
“As a statistical unit in this Encyclopedia, a ‘denomination’ always refers to one single country. Thus the Roman Catholic Church, although a single organization, is described here as consisting of 236 denominations in the world’s 238 countries.” (Barrett, et al, World Christian Encyclopedia, volume 1, page 27, in the “Glossary” under definition for “Denomination”
So if you process the numbers to eliminate this country effect, the Catholic church is effectively 1 denomination. I do not know how to process the Protestant numbers since I do not know whether how Protestant churches are structured in terms of reporting structure. If a church in country A roll up to another church in country B, you could essentially reduce that denomination count by 1.
However, perhaps the definition of “denomination” need to change. The existing definition seems pretty loose i.e. “organized aggregate of worship centers or congregations of similar ecclesiastical tradition”. The problem in my view is “similar ecclesiastical tradition”. What tradition? If the tradition follows the pastor, then you don’t really have a tradition as it dies out when the pastor passes away or moves to another church.
However, I’d love to see statistics grouped by doctrinal beliefs such as Abortion, homosexuality, Mary, Eucharist, etc. Then we will really know how Christian we really are.