According to their pricing[1] their lowest price is $19.00/mo. Assuming the bulk of their customers are on this price band it's roughly 3000 paying customers, paying each month.
They had it at $10 a month for a long time for people who started using it early when there were not a ton of features, myself included. So there could be a much higher number than that.
I cant recall it being cheaper than $10.00/mo. If you're looking for something cheaper than ghost there is Posthaven[1]. I'm not affiliated with Posthaven in any way, I just wanted to offer an alternative if anyone is looking.
[1] https://ghost.org/pricing/