You could make the hash, say, 2 bytes long. That would allow for up to 65536 servers.

Say you have 8 servers at present, you'd allocate 8192 sequential keys to each server.

If you need more capacity, use 16 servers instead, and move half the files from each server to the corresponding new server.

Thus exactly half the data is moved each time the number of servers is doubled or halved.