Amount of light by an LED
To find the luminescence (in lumens) of a 21W bulb, you can multiply the power in Watts by the luminous efficacy (lm/W). 
The efficacy for a typical incandescent bulb will be about 15 lm/W, so you're looking at about 315 lumens. 

Then it seems you are trying to find the amount of LEDs that will provide the same luminescence. If the LED's datasheet has the luminous efficacy, you can use that. If it lists the luminous intesity in candelas (cd, or LED's are often also measured in mcd), you can use the relation Lumens = 4Pi * Candelas. The value 4Pi assumes the emission of light is uniform in all directions.