Very unlikely you would need more than 2TB of the cache.
But I hope this helps:
Read and Write cache -2x 1TB cache would require 416MB RAM Read-only 832MB of RAM
Read and Write cache -2x 2TB cache would require 832MB RAM Read-only 1.7GB of RAM
Read and Write cache -2x 4TB cache would require 1.7GB RAM Read-only 3.3GB of RAM
None would need more then 4TB of cache, but you can have more if you add additional RAM stick,