You'll need a OneUp Switch carrier to use these chainrings. Once you have the Switch carrier choosing the chainring is the easier part. Round or Oval?
Oval can feel different at first but basically it evens your pedal stroke by balancing pedal torque through a higher tooth count. This is what gives it the extra "traction", while it technically does not increase traction its more like traction control.
Oval are not for everyone but will benefit riders climbing technical trails where traction is key to clearing it. It also works great for flat pedal riders as they are reliant on the hip drive and downward stroke more so than than the upstroke.
OneUp has done the smarts for you and the Oval rings are timed right at the sweet spot so you get all the benefits even if you ride clips or flats.
How many teeth do I need?
What ever you are riding at the moment choose that same tooth pattern. On the downstroke it adds 2 tooth and reduces by 2 teeth on the upstroke