When you say that a DNA or RNA strand is 20 bases long, does that mean that it is 20 base pairs long, or 10 pairs of two bases long? There is a reference to primase being 20 bases long, but that seems somewhat ambiguous.
If the nucleic acid is single stranded, and we say it is X units long, we mean X bases; if it is double stranded we usually mean X base pairs in the double stranded molecule or X bases in each strand (In your example, the DNA contains 20 pairs, not 10 pairs).
By the way, primase is an enzyme; what it makes is called primer.