A 20 gallon long tank is not really suitable for angels. Angels grow "tall" and thus need a tall tank. At least 18" is the minimum I would use for angels. Plus, angels grow large in general, so maybe 1 for a 20 gallon high tank, though a larger tank is best. I amnot sure however what is the largest fish you can put in a 20 long. I just know that angels are not good for one.
\Dan