I'd translate it as follows: "Huh? I'm sure it should be 20 participants, but 21 are here."
I interpreted "should" because のに to my knowledge indicates a surprise here. Using plain indicative wouldn't reflect that in my opinion.
However, the real "problem" is the bold な. First, I don't know into what category はず falls (formation morpheme like suffix, or anything else like noun, verb etc.). Therefore it is hard for me to try to use other grammatical rules to deduce the meaning of な, like な after nouns in -んです/-のです constructions. And even if I knew that, I still wouldn't know what function it bears here.