The number of repetition of the Beta oxidation requires to fully convert 18 carbon activated fatty acid to Acetyl COA molecule is NINE.
Beta oxidation of fatty acid involves oxidative removal of successive two carbon units in the form of acetlyl COA, starting from the carboxyl end of the fatty acyl chain. Thus, a fatty acid which has 18 carbons will have to undergo nine repetition of beta oxidation before it is reduced to an acetyl COA.