Now there are a lot of ways to answer that question:
1. How many people were willing to join as a lay member of the Seven Mothers? That might be a big number. Even Orlanth has no real restriction on that – any non-Chaotic sentient that breathes Air may join the Orlanth cult, as long as they are not already an initiate of a Neutral or worse cult. So an Orlanth initiate can be a lay member of the Seven Mothers, no problem. Even easier for Ernalda initiates. This could have been more people than you might imagine, even in Sartar, especially in 1613-1621.
2. How many adults already initiated into another cult were willing to initiate to the Seven Mothers? If you weren’t already from a family of Seven Mothers cultists or their affiliates, this might be a small number. Let’s go to Ernalda. She’s Neutral towards the Seven Mothers, which our Ernalda initiate would need to leave the Ernalda cult (and brave the consequences) to join the Seven Mothers. That’s a pretty high burden and so she would need to really really have a good reason to do that, more than just convenience. Orlanth is even worse – he’s hostile towards the Seven Mothers. So we are talking very small numbers here, maybe a single percentage point at best.
3. How many young people forgo their traditional adulthood rites to initiate to the Seven Mothers? This might be a bigger number than 2. We have plenty of orphans, destroyed kinship groups, etc., where this might be an easier option than the traditional Orlanthi initiation rites, even in the cities. This might be a couple of percentage points or so.
So where did the Lunar initiates in Sartar come from? Most were already Lunars, from the Heartlands or from the Provinces.
Is this different in the Alda-Chur tribes like the Dinacoli? Not really. For an initiate to leave their cult and becoming apostate is costly and even dangerous. You need to have REALLY compelling reasons or be able to afford the spiritual protection. And the Seven Mothers isn’t better than Neutral towards any traditional cults in the area. So that means going apostate to join.