The default block replication factor (dfs.replication) is three. Replication affects disk space but not memory consumption.
Replication changes the amount of storage required for each block but not the number of blocks. If one block file on a DataNode, represented by one block on the NameNode, is replicated three times, the number of block files is tripled but not the number of blocks that represent them.
With replication off, one file of 192 MB consumes 192 MB of disk space and approximately 450 bytes of memory. If you have one million of these files, or 192 TB of data, you need 192 TB of disk space and, without considering the RPC workload, 450 MB of memory: (1 million inodes + 2 million blocks) * 150 bytes. With default replication on, you need 576 TB of disk space: (192 TB * 3) but the memory usage stay the same, 450 MB. When you account for bookkeeping and RPCs, and follow the recommendation of 1 GB of heap memory for every million blocks, a much safer estimate for this scenario is 2 GB of memory (with or without replication).