Even if you are measuring prevalence by measuring the total population, you are statistically sampling from a hypothetical universe of the variable considered. Hence ,estimating precision of the estimate requires sample size considerations..

If you want your computed prevalence to be representative and applicable for others to cite and use, you have to ensure that there is a minimum sample size (effect size). This applies to all studies not just prevalence studlies.

You can use formule of Schwartz