I have seen a bunch of estimates by Kashikar et al, Kannan and via various personal contacts. Drawing from all of them I come up with some numbers. These numbers can only be considered the rough lower bound
Tamil Nad (smArta-s and srI-vaiShNavas): 145 (found also secondarily displaced to Kashi e.g. Purushottama Dravid, a great soma ritualist)
Kerala (nambUthiris): 125
Karnataka (mainly havayaka and some tulavas): 50
Andhra (mainly vaidIki smArta-s): 160
Maharashtra (deshashtha, konkaNastha and karhADe): 100 (more than half for them live outside mahArAShTra in Kashi); Maharashtra also had the only shUdra who had performed a shrauta ritual in recent memory.
Rajasthan (brAhmaNa-s and kShatriya-s): 10 (Rajasthan is the only place were royal AhitAgni-s existed until recently. Some brAhmaNas of Rajasthani origin live in Kashi)
UP (mishra-s and other native UP brahmins): 8
Some say that the greatest modern ritualist is the Andhra smArta: Renduchintala Venkatachala Yajulu. He performed numerous extraordinary and rare rites. Amidst the smArta-s of my ethnic group Narayanaswami Dikshitar, also udgAtar, a clansman on my maternal side was the greatest recent performer of rare and complex yAga-s.