A.
For using Table 10.5.C, you must follow subparagraphs (a) through (f) of DGR
10.5.15.1. (c) reads: “if the Transport Index is greater than 10, the packages or overpacks must be transported under exclusive use (refer 10.5.17).

A single package of TI 1.0 is categorized as II-Yellow. A shipment of 20 individual packages of each TI 1.0 is still Category II-Yellow. But, if you place the 20 packages of each TI 1.0 into an overpack, you have two choices to determine the TI of the overpack, i.e., as stipulated in 10.5.14.1.2 “Transport Index – Consignment” where “the TI for each overpack or freight container must be determined as ether the sum of the TIs of all the packages contained, or by direct measurement of radiation level, except in the case of non-rigid overpacks for which the TI must be determined only as the sum of the TIs of all the packages.” The maximum radiation level at any point on the external surface of the overpack must be determined by direct measurement.

According to Table 10.5.C, if the package or overpack has a TI of more than 10 and the maximum radiation level at any point on the external surface of the package or the overpck is more than 2 mSv/h, the category is III-Yellow but (**) must be transported under exclusive use and special arrangement.
If a package or overpack shows a maximum radiation level at any point on the external surface of the package or overpack exceeding 10 mSv/h that package or ovepack can not be forwarded by air but must be transported by surface (ocean or land) transport.

I may add that radioactive material shipments are seldom overpacked. They are more or less shipped as individual packages