It does not. It in large part scales with the amount of content the local and federated communities. So because many users will subscribe to many of the same communities, more users will add less extra cost aftter a point. This is a bit simplified though as it also scales with users but to a lesser degree from my understanding.

Disclaimer: I’m not hosting myself just to be clear but this is what I have gathered from answers to similar questions as well as my own experience as a software dev.