Performance-wise, 100 hosts per BackupPC server shouldn't be a problem. I have 1 BackupPC server backing up ~100 other Linux machines, and it manages to keep up with performance, with compression enabled. If compression was disabled, I reckon I could backup even more clients.
That thing you mention for separate "disk" or "partition" per client though is unusual... The first consideration is that deduplication won't work (you state that you don't need it anyway) because BackupPC won't be able to create hard links between the files of each client. Other than that, I think that backing up each host in a different directory is contrary to the basic logic of BackupPC, and not possible.