Based on the docs and look at the processes, it seems 1 wal sender on each node per group.
If there is scenario of consolidating many databases (say hundreds) into 1 database (in this central cluster there are hundreds wal sender), what is the limit number of groups?
It's not a use case I've paid much attention to. I expect it'll be limited by performance and memory, rather than have any firm limit.
Maybe you should look into pglogical. This seems like a weird use for BDR.