You should only need one, but if you feel you need more, two won't hurt.
Don't know what Don was referring to, but you didn't mention the WDH at first. Maybe that was it. But also, I thought I might need two sway bars until I messed around with mine and found I didn't have it set right. Made all the difference.
Otherwise, with a WDH and sway control, if you feel like you're still experiencing a lot of sway, check your load distribution. When I started, I didn't know anything, and loaded everything in the back of the trailer thinking less weight on the hitch the better. I had way too much in the back and experienced a lot of porpoising and a bit of sway. Got educated, and all was well soon after.