Substack's publication subscriber numbers have been growing rapidly, growing 3 to 5x per year. How much will that growth slow, if at all?

The last information Substack has released is that their platform first supported a million paid subscriptions as of November 2021.

I'll wait up to a year after the market closes on verified subscription numbers to resolve. If nothing has been released by then, I'll estimate to the best of my ability, based on confirmed or estimated information, like revenue, free subscriptions, pageviews, and the performance of individual substacks with public information.

Edit: I've extended the market to be open until right before I resolve, but to be clear 'by 2025' refers to the end of 2024.