adding a totals sheet

The most direct way is to add each of the 12 cells.

=January::Table 1::B20+February::Table 1::B20+...etc

Another way involves a summary table:

Text in column A and row 1 is typed in manually.

Formula in B2 =INDIRECT($A2&"::"&B$1)

Fill down for the rest of the months

Fill right if you have other cells to collect from these sheets

Formula in B15 (footer row) = SUM(B)

Your Totals table will then refer to that sum.

You don't really need this footer formula, you can SUM them in your Totals table or do averages or whatever other manipulations you need to do on the 12 values.

One thing about the table with the INDIRECT functions is that they will not adjust like regular cell references if the cell moves (such as if a row is added above). INDIRECT("January::Table 1::B20") will be that exact cell and stay that exact cell.

There is a different way to do the table method that uses the "header names as labels" feature of Numbers. Instead of "Table 1::B20" in the header row of the summary table you use the label for that cell. If in the month tables the column B header has "1" and the Row 20 header has "Total" then it would be "Table 1::Total 1". This method will point to whichever cell in Table 1 has the label "Total 1". You cannot have two columns with the name "1" or two rows with the name "total" or this method breaks.

For only 12 additions it might be simplest to just add them the usual way vs using the table method.

Yep, that will work as well.