What temp are you planning to ferment at? I spoke to CML about this a while back, and right down at the bottom end of the range (12- 14 degrees) two packs might be advisable in a 23L batch, but higher up one should do. Its cheap as chips, so if you are concerned, pitch two.
I pushed this right down to 12 degrees, IIRC, with 2 packs and got an extremely clean, lager like result, using an fairly simple historic recipe.