If you are asking, "How many lemons should I be able to harvest from my tree?", that will be very difficult to answer. There are to many variables involved with fruit production to give a clear-cut answer. For example, the size, age, type tree, weather, soil, fertilizer, water, sun, pollinators, temperature and tree health all play a role in determining how much fruit will mature on any plant.
This link is for an article published by Purdue University, and discussed lemons at length: http://www.hort.purdue.edu/newcrop/morton/lemon.html
I hope this helps answer your question.