There are multiple methodologies out there - if you are trying to identify the formula for sample size calculator which Adobe uses please consider this:
x : Baseline Conversion Rate (control offer)
y: MDE
z1: Z score of Confidence Level
z2: Z score of Statistical Power
=2*((x*(1-x)+(1+y)*x*(1-(1+y)*x))/2)*(z1+z2)^2/(x-x*(1+y))^2