Generally you would use one light per 30,000 to 40,000lts. A standard 60,000lt pool requires two lights to be effective. They are best placed on the long walls of the pool for maximum spread of light. There are different sizes and most have different coloured lenses. These days the output has been restricted to 12 volts for safety reasons.
This type of light is a little more expensive but is a better quality of light and is best used if your pool is to be "on display". It also has a wider variety of uses such as illuminating waterfalls and water features and is more of an aesthetic light rather than an illuminating light..