Merge Simulate can tell you the number of output tapes needed for Live run.
You can run MERGE SIMULATE to help determine how many output tapes would be needed by specifying a special value for the output device capacity.
Example: If you are using the DYN1 path (sysparm MERP1TYP set to DYN1) then specify the special simulate value that relates to your tape capacity with the DYN1UNIT sysparm. (If DYN2 then specify DYN2UNIT etc.)
The simulate values are documented in the Reference Guide, section "System Parameter Definitions", in the entry "DYNnUNIT".