Here’s research of 25 years ago funded by the National Science Foundation that references no less than 154 Protestant denominations in the US alone.
http://www.adherents.com/largecom/prot_classify.html
And they also included the cults in the count
Sometimes one becomes apostate and then the traditional members break off as what happened with the PCUSA or more recently the ELC went liberal crazy so the remnant are forming the northern Lutheran denomination
They will basically hold to the same doctrine as the Missouri Synod
Denomination are often regional or racial or ethnic at their core not doctrinal
BTW 154 is a LONG way from 20,000