Scaling is a big concern with Lemmy, and pretty much any Fediverse platform. However the positive side is the software can adapt and as it becomes more popular it will naturally acquire more resources.

Admins do have control over the size of their instance. They can shut down new sign-ups if they get to a point where they can't support any more users. They can either expand their capacity or leave sign-ups closed. Ideally as you get more users you get more support through donations which can cover the cost of expansion.

Another aspect is how well ActivityPub can hold up to demand as more and more instances enter the fold (ActivityPub is the protocol standard that supports the Fediverse). Hopefully the designers have put a lot of thought into scalability. ActivityPub has mechanisms to limit instance resources to what's on demand from users of that instance. It's not like each instance has to accommodate the whole of data and bandwidth comprised by the platform.

As users we'll just have to see how it holds up and have some faith the designers did a good job with scalability. I think the other aspect of having enough instances to support the size of the user base will work itself out naturally.