They all do good work, there are others, but these are the ones I'm familiar with along with their membership numbers.
According to the NRA, their membership reached 5.5 million total members in 2018, a record high
https://en.wikipedia.org/wiki/National_Rifle_Association#Membership
GOA claims >100,000 members
https://en.wikipedia.org/wiki/Gun_Owners_of_America
The Citizens Committee for the Right to Keep and Bear Arms (CCRKBA) is the lobbying affiliate of the SAF. As of January 2015, both groups reported having over 650,000 members
https://en.wikipedia.org/wiki/Second_Amendment_Foundation
Forgot JPFO, Alan Gottlieb, CEO; last reported membership number I could find was 7000.
Texas State Rifle Assn.
Founded in 1919, the Texas State Rifle Association is the largest firearms and shooting sports organization in Texas with 40,000 members. From article dated
2014:
https://www.ammoland.com/2014/10/te...ation-names-dubois-as-new-executive-director/
The total membership numbers of the four most recognized allies is around 800,000.