The range for COPY is "number of elements", hence, the element designated (as casted) is a "double word", so 1 is correct.
If you want to see for sure, try entering 2 for the range, and in the Ladder Display of the COPY instruction, you should see something like
Source: V2030:SD - V2032:SD (range of 2 DWORDs)
Destination: V2040:SD - V2042:SD (range of 2 DWORDs)