What is the Statistical formula to determine the amount of surveys one must implement in any given population?

I know there is a Stat formula for this, I just can't remember it and I Can't find it anywhere. Please if someone knows, let me know.

What is the Statistical formula to determine the amount of surveys one must implement in any given population?
For measurements:


n = [Z(sub α/2)σ/E]² Where Z(sub α/2) is the standard normal variate found in the table for α/2. σ is the assumed standard deviation of the population you are sampling. You might have to guess at this. It greatly affects your sample size. E is the margin of error you are willing to accept.





For proportions:


n = [Z(sub α/2)/E]²(p)(1-p) Where p is the assumed decimal fraction of the proportion of the population.

shoe labels