simple answer
In a SD game, you want the best penetration possible. Take a more normal Ro6 game:
you play 1 hand, heads up. You get 5 rounds, which is 10 hands counting the dealer's hand, or 27 cards dealt.
you play 2 hands, heads up, you get 4 rounds, which is 12 hands counting the dealer's hand, or 33 cards.
you play 3 hands, heads up, you get 3 rounds which is again 12 hands counting the dealers hand, or 33 cards.
you play 4 hands, heads up, you get 2 rounds, which is back to 10 hands counting the dealer's hand, or 27 cards.
You play 5 hands, heads up, you get 1 round or 6 hands, which sucks completely.
I always play 2 hands (if possible) in Ro6 games. Only time you might choose to not do this is if the table min is at your lower betting limit, and they make you bet 2x the table min if you play 2 spots. That could over-bet your B/R and increase risk unacceptably.
This only affects the RoN games (I have seen N=6 frequently, 7 on a few occasions, have not played N=5 nor have I seen it anywhere.)
I would also choose to play more than one hand at a crowded shoe game should the count go up, to try to greedily eat more of the good cards in the + count...