Nope, ActivityPub (the protocol) doesn't support communities to be distributed over multiple instances.
Instances can horizontally scale to multiple servers, just like massive websites like reddit do. If you host a huge community, you can gather enough donations to pay for the hosting of your scaled instance.
I can imagine that a future version of ActivityPub will support something like grouping communities of different instances, which could also allow scaling, but will be a bit awkward given you'd then have a community run on multiple instances, so with potentially different rules and the possibility of communities splitting when the instances decide not to group or federate anymore.