This refers to the number of LEDs that the lens is designed for. They can be for single LEDs or arrays of LEDs. Multiple LED lenses are ideal for wide area lighting, for example street lighting.
Lenses are often designed for use with specific LEDs. Check the manufacturer data for compatible LED options.
Some manufacturers pre-mount LEDs onto a board ready for the lens/reflector to sit over, saving you having to solder and line the LEDs up accurately.