The debate about the appropriate number of treadles has gone on for decades. Some spinners say you really only need one; others find two more comfortable. There is no right answer, but to help you find your preference, here are a few factors.
When I bought my first spinning wheel, I was a long-distance runner and very prone to repetitive strain injuries. I was probably not going to get leg pain from uneven treadling, but I was not taking any chances, either. (I also alternated directions running around a track so that I wouldn't be affected by one leg moving differently than the other.)
And it made sense, in a way! There's an instinctive urge to put one foot in front of the other, to share a perceived load between two feet. Two treadles felt like riding a horse astride; one treadle felt like riding side-saddle.
But for hundreds of years, the primary form factor for a bobbin-and-flyer spinning wheel was a single treadle. A second treadle seems obvious but isn't usually necessary.
Points of Contact
If you are attached by only one foot to the spinning wheel, the rest of you can move around in the most comfortable, efficient way you need. (Sure, you're also connected to the orifice by your yarn, but it's flexible.) With two treadles, you pretty much face forward.
This is one of the biggest appeals of the e-spinner: with no treadles, the bobbin and flyer can sit wherever you like. You are not tied to the wheel except by your yarn.
Charkhas and great wheels don't have treadles, but instead of giving the spinner more flexibility on where to sit or stand, they give less: One hand has to operate the wheel, the other hand has to make the yarn. I can imagine wearing a trough in the floor spinning yarn on a great wheel, walking back and forth in the same rhythm over and over.
How do you choose?
Well, what's your favorite draft? That may give you a clue.
If you like the sweep of a long draw, it may feel good to swivel in your seat to draft back; try it on a single-treadle spinning wheel and see. If your hands stay mostly close together as you short draw, the facing two treadles head on might feel right.
When I bought my first double-treadle wheel, my hands were strong, and I loved the precision and symmetry of short forward draw. My feet did a brisk tap-tap, my hands pinched and slid, and I made miles and miles of smooth worsted yarn.
As time has gone by, I've found more joy in spinning woolen yarns than I did at first. I'm more comfortable with trusting the twist and my hands to keep the yarn continuous. I often reach for my e-spinner, because I can draft almost perpendicular to the orifice and keep from cramping up.
Maybe the answer to how many treadles one spinner needs is like how many spinning wheels a spinner needs: Just one, mostly, but why stop there?