Mulcarn never had many followers. That is a big part of why he chose to invade and wipe out those ungrateful civilizations. The Illians were once of the Patrian factions, but they had always been the smallest and weakest. They were probably only kept alive during the civil war by the fact that no one really wanted their barren land. Mulcarn was not seen a a real threat in the Age of Magic, so his priests were generally tolerated everywhere. However, they were horrible at evangelizing so they never had large numbers.
It is probably true that all the gods had at least a few worshipers dating back into the Age of Dragons, but the religions are rather new. We know that Immanuel Logos was a priest-king of Sirona, but that he gave up this position in order to tend to the orphans of those of all faiths. Nominally he may have ceased to serve Sirona, but in doing so he more fully exemplified her precept than he had before.
Sucellus was worshiped by all Elves, and Danalin by all Aifons. Once the Dwarves were created, they all worshiped Kilmorph. Each of these gods probably had a few human worshipers as well.
The Worship of Ceridwen was really big in Patria, once Kylorin fell and made this the state religion. Before this, they had been more philosophical than religious. Cassiel had made his home among them, and was widely revered. Ceridwen's cult was mostly a religion for the upper class, so commoners probably remained non-religious or followed other gods, especially good ones.
Once Patria began disintegrating, Bhall worship really took off, as her power was the most effective against the sorcerers. Fire was a holy weapon at the time, which could be used indiscriminately against innocent and guilty and cause no harm to the innocent. It was judge, Jury, and Executioner. (Many sorcerers began praying to Danalin, hoping that this would help them better channel the power of water to combat the flames that would consume them). She had far more worshipers than any other god, maybe as many as all the others combined, at the time of her fall. This worship was centered at her great temple at the heart of the Bannor capital of Braduk, where the Eternal Flame was kept.