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.
just a wild a guess. will it should be more dependant on bandwidth and traffic than users. posters on c/cats are more demanding than posters of c/news in terms of ressources and bandwidth. text is lightweight compared to image or gifs.
For sure. Larger userbase also increases likelihood of more and bigger donors, however, so the critical question is whether that revenue increases at a higher or lower rate than server costs.
users just need to be ressource conscious: no wasted bandwidth with 4k pics and we should all fare well. sharing 70kb memes here and there shouldn't hurt thou. but good practice is to use a dedicated media hosting site and use links instead of posting media directly (like imgur or giphy). peertube could help in hosting videos in case people felt like sharing some.