Five is actually doing pretty well. And, he's not saying this is the best case scenario, just the likely scenario at this point in time.
It's just not a great year for GOP opportunity in the Senate this cycle because - as luck would have it - there are more Republicans (or vacated GOP seats) up for reelection than there are Democrats.
Remember, only 1/3 of the Senate is up for reelection in any one election year and not the entire body, like is in the House.
If JD beats McCain, you can all it six.