In my text book there is an example of a 18 carbon long fat and it says that it can run 8 laps of oxidation. I don't understand why it doesn't run 9? 2 are removed each cycle?
I believe your textbook is referring to the fact that you only perform the complete beta oxidation eight times. Because there's a carboxylic acid at the terminus of a fatty acid chain, the cell takes a slightly different route and reacts it with ATP, which generates a fatty acyl adenylate and pyrophosphate (PPi). This AMP can subsequently be displaced in an attack by CoASH, at which point the beta oxidation process can proceed in full to begin cleaving off acetyl-CoA.
Here's a visual representation of this activation process, too.