Yes - you're supposed to have 24.
Now you're checking such things, it's also worth making sure you have all the right player and infection cards.
Being paranoid, I made sure of such things during an initial unboxing of my set before I gathered players for the first game.
Yes I've just checked the rest of the components, we seem to have all the cards (or at least in the numbers they should be )
Like John said, probably a packaging error. I've put the extra cube away now. Glad I noticed before it impacted our games in any way.