It sounds like you're making the classic "newbie" mistake of using a Table like a Spreadsheet. If you're really interested in setting up a normalized relational database I'm sure people here would be happy to help you out!
That said, your current approach can actually be made to work. I'd strongly recommend however, that you always calculate your number of "Y" weeks "on-the-fly" since it's constantly changing. With the speed of today's computers (even last decade's computers), this can be done almost instantly.
All you have to do is add the fields together using an IIf() statement. In the Control Source of your Form/Report, do the following:
Code:
=IIf([week1]="Y",1,0)+IIf([week2]="Y",1,0)+IIf([week3]="Y",1,0)
Just replace [week1], etc. with the actual field names used. If your field names were Jan1, Jan2, and Jan3 for instance, you'd use:
Code:
=IIf([Jan1]="Y",1,0)+IIf([Jan2]="Y",1,0)+IIf([Jan3]="Y",1,0)
If you're going to be saving the number to your Table though, I'd recommend using aje's VBA solution instead.