you need to use =IMPORTRANGE
for that... 20 times and wrap it in =SUM
this is how =IMPORTRANGE
works:
=IMPORTRANGE("1W8yfNmNBBZuEKqk56k-ZxaKrLOiGOBASrkRCQNVxOSA"; "Sheet1!A1")
where the "code" from first quotes is ID of the spreadsheet (which can be found in URL of the particular spreadsheet) and 2nd phrase in the quotes is a combination of sheet name and cell reference/range.
the ID 1W8yfNmNBBZuEKqk56k-ZxaKrLOiGOBASrkRCQNVxOSA
is from:
https://docs.google.com/spreadsheets/d/1W8yfNmNBBZuEKqk56k-ZxaKrLOiGOBASrkRCQNVxOSA/edit
so basically you need to modify this formula by adding ID and sheet name:
=SUM(IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41");
IMPORTRANGE("ID"; "'sheet name'!C41"))
NOTE: you may be prompted to allow connection between those spreadsheets of yours...