If the percentages are accurate, most people have a 40% hatch rate from the eggs they set. (yes there are exceptions!!) So if you set 10, expect 4 to hatch. From those, expect 2 to be cockerells, and 2 to be pullets. Once they hatch, there is a mortality rate as well. If you go by a 50% mortality, chicken math works out for 1 pullet per 10 eggs.
No matter how many eggs you hatch, you have a 50 / 50 chance of getting a pullet.
No matter how many eggs you hatch, you have a 50 / 50 chance of getting a pullet.