That depends on what you really want. Most people prefer two lights in that situation. One light will work, it just won't be as bright as you are likely to want.
Placement depends on how many lights and how the pool is laid out. If there is a specific side you normally view the pool from it is often best to put the light(s) on the wall on that side, so you are not looking directly at the light. With one light, most people put it on one of the long walls somewhat centered but closer to the deep end. Wit two lights you can either spread them out along a long wall or put one on each of the short walls.
