I have approx 150 sheets named with my customers name. I want to create a
cell that shows the sum of C12 in each sheet. Each of my clients has a
number in C12 that shows the "Hours Required". I am trying to obtain a
total of all hours for all clients.

='Doe, John'!C12+'Smith, Jane'!C12 Works fine for a few sheets but what if
I have 150 sheets? What is the formula?
I have also tried =SUM('Doe, John'!C12:'Smith, Jane'!C12) This gives me
"$VALUE!"