How much lemons are needed to produce pure 1 kg lemon essential oil?
I haven't personally tried making lemon essential oil. But according to Young Living, an MD named Jean Valnet estimated that it takes 3,000 lemons to produce one kilo of Lemon essential oil. Here are some links to the estimations (keep in mind that a kilogram is roughly 2.2 lbs):


Young Living (states 3000 makes 1 kilo)  
LemonMania (states 3000 makes 1 kilo)
Veriditas Botanicals (states 3000
makes 2 lbs (about .91 kilo))