Let me give you the answer one by one.
1) For dry ice, regardless of whether it is a package or an overpack, even if there is only one package or one overpack. UN1845 Dry Ice and the net quantity must be indicated on the outside of the package or overpack.
2) Even if the contents of the multiple overpacks are exactly identical, if you make more than one overpack, the identification marking is needed for identification. Please refer to DGR Figure 8.1.N - Example 10 where 4 identical overpscks have been made and Identification numbers #1 through #4 are assigned.
3) When an overpack contains more than one UN numbers, the net quantity or gross weight, as applicable, is necessary to be shown on the overpack per each UN number. Even when multiple overpacks are made, even if the contents are identical, the net quantity or gross weight, as applicable, is needed for each UN number.