Which size knee pad to choose? And how many?
Either size fits most all people. Whether you are tiny/scrawny/spindly 'little guy' or a tree trunk legged big kahuna either will usually fit. That being said, often those two extremes will choose the Mini and the Large respectively, but not always. Plenty of big people wear the Mini, and lots of smaller folks choose the Large. For everyone in between it's really more of a preference.
The knee pads are a wrap around design - thus they are adjustable. You can wrap them tight to your leg and cinch it down, therefore they will fit most sized legs. The size difference is mainly in rubber coverage and number of straps. People who choose the Mini enjoy its unobtrusive, minimal design, with its shorter rubber and faster 2-buckle closure. People who choose the Large enjoy its large rubber coverage, and very secure 3-buckle attachment to the leg.
As far as how many, often people buy one, sometimes one of each size, or a pair! It depends entirely on your use. For sport climbing two are typically bought as you don't always know all the kneebars on a particular climb until you get to it. Many boulderers are also now choosing 2 because more and more kneebars are being found on problems.