There’s about 19,000 people in Sun County, with some 4350 initiates of Yelmalio centered on the Sun Dome Temple. Throw in another 1000 Yelmalio cultists and lay members in Pavis County, and another 1000 Aldryami (few are Yelmalio initiates but Yelmalio is an associated deity for them).
There are actually more Yelmalio cultists among the Impala and Sable people, as part of a cluster of deities that includes Waha, Eiritha, Storm Bull, Orlanth, Daka Fal, Chalana Arroy, Issaries, and even the Seven Mothers. Yelmalio is worshipped independently as the god of Light and Truth. He upholds agreements and contracts.
For those wanting a good real-world analogy, look to the Indo-Iranian Mithra (not the weird Roman version, but the old school god of the Iranian tribes, Indo-Aryans, and Mitanni).
Let’s imagine the Impala nation as a whole. Their principal deity is Eiritha, and then we have Waha, Orlanth, and Yelmalio after that. Daka Fan ancestor worship forms a third tier, and after that are clusters of Storm Bull worshipers, Seven Mothers cultists, Chalana Arroy healers, Issaries traders, and a smattering of even smaller cults.
The Sables, post their defeat in 1624, have Waha as their principal deity, followed by Eiritha and Yelmalio. Daka Fal and Seven Mothers are the next tier (prior to 1624, the Seven Mothers were in the same tier as Eiritha and Yelmalio), followed by smatterings of other cults.
In both of the nations, Yelmalio is the God of Wide-Pastures, the All-Seeing, and associated with truth and celestial light (the stars, the dawn, etc). They recognise their co-religionists of Sun County, but do not consider them to be their spiritual leaders.
And in Peloria and Pent the God of Wide-Pastures can also be called Kargzant, while as the God of Truth and Justice he can be called Antirius. But it is all Yelmalio.