The picture and description are ambiguous -- is this product a set of 2, or do you have to order in quantities of 2?

Legitimate question, I was wondering the same thing. Since everyone has confirmed that there are two, then they should update the Technical Details to state there there are 2, since it currently says that the number of items is 1.

I'm a women's size 6 wearing a MTB shoe- I'm not sure what size to get. Suggestions?

Just an FYI, the saddle position is centered when you can ride your bike hands free without antics. If you are out of balance than it's off center. Any modern bike fitter who is properly trained can verify this is a simple (but perhaps not absolute rule a FITS YOU) rule in terms of saddle balance. The plumb line KOP (knee over pedal) is old school and now incorrect as more nuanced fitting has evolved. Again I'm not saying your bike fits because you can pedal seated no hands, I'm saying your saddle is centered. Now is your knee is the correct position when your saddle centered? That is determined by if your bought the correct size bike frame. That's where going to a good bike shop and perhaps a bike fitter before laying out $5000 is helpful. But most folks outsmart themselves and do it commando. $5k later they leave watts on the bench because of poor bike fit. Proper fit = optimal output. More expensive bike doesn't always.

Do these work with fitness bike?

If your fitness bike has a standard bicycle pedal, they will work.

I'm womens 9.5. What size will work? Please note: there is no M/L size to choose from as in previous answer.

A women’s 9.5 is equivalent to a men’s 7.5-8, you should use the S/M according to the package. I am assuming they’re talking about a men’s size 9 to hen they indicate less than 9 go with the S/M, greater than 9 go with the L/XL.