The number of lights required depends on variety of factors.
Some of them are-
Size and height of the room– Normally for a room of 4×4 m² with a ceiling height of 2.5 m you need 4 downlights of 10watts for good lighting. As the height and size of the room increases the light intensity also reduces.
Individuals requirement– Some people like little light and some people like real bright light all the rooms. So you can go from 1 to 5 lights for a 4 m² room.
Light intensity – This depends on the wattage and lumens of light they give out. For a 10 watts light normally we can expect around 900 lumens. This varies from manufacturers of light.
Colour of the room – the painting of the room also plays an important role in the lighting of the room. More darker the colour, the light looks dull. This means white colour will be the brightest for the same light used at both places.
An electrician will give you a better idea and estimate of the amount of lights required.