There's no quick and easy answer to your question, quilt lady1941, because there are any number of ways to organize and arrange a log cabin quilt. I suggest you type 12" log cabin quilt block into your search engine and read through enough of the suggested websites to get a good feel for what you plan to do.
Once you've done that, you'll probably know what width your strips will need to be. I'm guessing they'll be about 3" wide, which will finish at 2.5". (Using 6 colors, I think that gets you pretty close to a 12" block.) Then just calculate the number of strip inches you need for each color, and compare that to the width of your yardage. Cut enough length of strips for each color. Note: The strip lengths will not be the same for each color!
I hope this helps.