Number of cores is not going to be your issue because “bulk uploads” = disk bottle neck with i/o. Couchbase is append only so there is really no CPU issues as the head of the HD never has to do seeks or find items to update.
Check your Write que growth if it keeps climbing you will need to back off.
Also check your write fill rate(ie how many items is the QUE) vs. your write drain(ie. how many items are really being written to disk.) They should both see-saw with each other.
Also lots of replica( 2-3 ) + only having less the five servers means more work your HD’s have to do.