List Manipulation: Add

Here is how I would do it:

makeTOC[names_, n_] := Flatten[
    Partition[
        Thread[{Range@Length@names, names}],
        n, n, 1, {}
    ],
    {{2}, {1, 3}}
]

And a few examples:

makeTOC[testNames, 3] //TableForm
makeTOC[testNames, 5] //TableForm
makeTOC[testNames, 8] //TableForm

one another possibility

make list of names

list = ("name_" <> ToString[#]) & /@ Range[13]

Then use Partition with option UpTo and then use TableForm with Heading

nRows=4;
TableForm[Partition[list,UpTo[nRows]],TableHeadings->{Range[nRows],None}]

Multicolumn is similar to what you are trying to do except that Multicolumn[list, cols] uses the smallest number of rows so that all elements will be shown. That is, for a given number of columns it balances the entries per column.

makeTOC[nameList_List, nRowsIn_Integer: 5] := 
 Multicolumn[testNames, Ceiling[Length[nameList]/nRowsIn]]

testNames = "Name_" <> # & /@ CharacterRange["A", "J"];

makeTOC[testNames, #] & /@ {3, 5, 8, 12} //
 Column[#, Spacings -> 2] &