I would imagine if the growth was too tremendous, the instance owners could always temporarily disable sign ups until more server infrastructure is ramped up.
I can imagine this happening after Reddit loses more giants like BrookValley and once all the mobile third party mobile apps contain all the Lemmy features. I remember in June of last year when all the iOS apps were in beta and Wefwef was the only option and god forbid you wanted to do some modding on the fly and now look how far we have come.
Time to start creating those communities baby!