Using countif to look at multiple sheets???

I am trying to create a formula that will tell me how often our sales reps are going to specific accounts.

I have a master list of each persons accounts and next to each account name there will be a cell with a formula in it. This formula will look at the cell next to it, "the account name", and then count how many times that name comes up over each months sheet. I was also wondering if I can then get the average over 12 months in the same formula. If not thats ok.

The problem I am having is that I keep getting an error #VALUE!. I had no problem with it looking at only one sheet, but when I do more than one it gives me an error. I think it is the was I am breaking up each sheet. I have tried to do it every way I can think of... ,;) Nothing seems to work.

Here is the formula that I am trying to use. What am I doing wrong?

=COUNTIF(('Apr-08'!E7:E114):('May-08'!E7:E115):('Jun-08'!E7:E109):('Jul-08'!E7:E121):('Aug-08'!E7:E111):('Sep-08'!E7:E116):('Oct-08'!E7:E121):('Nov-08'!E7:E106):('Dec-08'!E7:E121):('Jan-09'!E7:E116):('Feb-09'!E7:E106):('Mar-09'!E7:E116),Customers!A20)

Thanks for your help!
Lukas