Hi,
The first choice would have been an INDIRECT() function but Excel has trouble when using the INDIRECT function with 3D ranges as you are doing here. The only way I know is as follows.
Create a list of your month tab names, presumably Jan,Feb,Mar etc.. in say A5:A16.
Now create a dynamic range name called 'SheetList' and define the name as
=OFFSET(Summary!$A$5,Summary!$B$1-1,0,Summary!$B$2,1)
In B1 enter
=MATCH(A1,$A$5:$A$16,FALSE)
and copy this to B2. These will return the month number.
Finally in C7 enter the following array formula, i.e. with Ctrl-Shift-Enter
=SUM(N(INDIRECT(SheetList&"!"&"C7")))
See the attached as an example workbook
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