AFAICS the function org-shiftmetadown is not implemented to handle the numeric prefix argument as a count of actions.
You can use a keyboard macro to use the prefix argument as a count for the number of repetitions.
One recipe which might do what you want goes like this:
C-x C-( M-S-down C-x C-) C-x C-k b F5 With the lines above F5 calls the macro (which does the M-S-down.)
The macro can handle numeric prefix arguments. E.g.
C-u 23 F5 calls the macro 23 times.