@WillowTree so I might have a more concrete answer! Sorry for the delay, my editor and I had a few e-mail exchanges about this.
So, this isn't a set in stone sort of answer but based on the experience of my editor. I still found it insightful.
For free-to-read series: A good milestone is 100-250 subs, but a lot of novels tend to get bogged down at the 300-400 mark but if the series reaches 1K, that is viewed as a success story for the platform.
For Premium novels: 2k means the story is doing ok. 10K is considered to have done very well and is the threshold that Tapas typically aims for novels.
There's still a lot of gaps, but given that novels are still fairly new to Tapas, it's to be expected. I hope that provides a little more detail for you!