Assuming you are using the SimpleStrategy or the NetworkTopologyStrategy and one rack per DC. If you backed up every 2nd node you would get one copy *IF* all nodes were consistent on disk. That can be a reasonably large if that you need to monitor.
It's easier to back up all the nodes it will also make it easier to restore the cluster.
Freelance Cassandra Consultant
Hi – If we have a RF=2 in a 4 node cluster, how do we ensure that the backup taken is only for 1 copy of the data ? in other words, is it possible for us to take back-up only from 2 nodes and not all 4 and still have at least 1 copy of the data ?