Honestly, the amount of characters you play and what you want to accomplish are going to go hand in hand.
Since you're planning to main shadow priest, I assume you're wanting to raid/pvp solo some. Given this, honestly I would not play 2 priests in your duo box set up. With shadow priest being required, I would suggest dual boxing with either a mage (free water, portals) or warlock (shadow buffing each other, voidwalker). For 3 boxing, mage and warlock would be very good as you get both of the gains. Once you get to a 3 or 5 box set up, especially in classic, certain quests become very time consuming. My personal opinion is, for a person playing mostly alone, doing either 2 or 5 boxing is going to be the best in classic.