I think you could probably miss ~8 problems and still end up with a 20. I'd say that memorizing Cliff's could get you that 20. However, I'd also recommend buying Barron's AP biology and memorizing it, too. They're mostly just overlap, but Barron's is an easier read and they both have some sections that are stronger than the other. Oh, and Barron's AP is cheap on amazon.
I used Barron's AP, Cliff's AP, and the Destroyer for biology. The Destroyer was overkill. I think I knew Barron's and Cliff's pretty dang well and they gave me what I needed to answer the questions. I scored a 30.