I would not do motion detector for arena lights–seems like they’d be going off/on constantly. And, the model you linked has the motion sensor built into the lighting housing-- so you’d need to mount them closer to the ground than you really want, otherwise they won’t “see” you as you go by.
How many and how tall will totally depend on the design of the fixture & and lumens of the lamps you select. Also on how you will be using the arena–general WTC on the rail and some figures will have different lighting requirements than jumping a course.
As part of the specifications of any given fixture, they’ll tell you how wide an area each lamp will illuminate. One rule of thumb is that mounting height = 1/2 distance to be illuminated (ie, the extent of the “pool” of light on the ground). This assumes the fixture can be aimed a bit (but it should not be angled more than 30degrees or so, otherwise you’ll have blinding glare). Anyway by this rule, if you have a 60 ft wide arena, and you want the whole riding surface illuminated, you’d need 30ft poles, which is unrealistic in a farm setting.