[newlyretired]

I doubt it was specific to the exact MRR number. More likely their growth stalled using the methods that were taking up all of their time (direct sales, word of mouth).

For some $N MRR (magic point where you look for scalable channels), N should be large enough that you have a product that your market wants and will pay for, and the market is large enough that you were able to manually scale it to N.

The heuristic is something like "if you can get big enough doing low-efficiency/manual marketing, you've likely gotten close enough to the important things* and can move on to testing scalability".

* things like product/market fit, market size, clear communication, well defined value prop, etc.