HI
try
=sum('Campaign 1 - blahblah'!C11:'Campaign 50 - blahblah'!C11)
or paste the following codes in the macro window(Alt F11)

sub total()
dim a as integer
cells(1,1) = ""
for a = 1 to sheets.count
cells(1,1) = cells(1,1) + worksheets(a).cells(11,3)
next a
end sub
Run the macro. it will give total in cell A1.
Ravi