Need help determining sample size


I've been asked to analyze a tool that gives out quotes to our customers. There was feedback that some of the quotes are wrong and the team wants to know how often this happens.

I found a sample size calculator online,, but I am having problems determining the population. Should I use a year's worth of data as my general population? Can I use a month's worth of data and extrapolate? Or should I use the average of 12 months worth of data as the population size?

Thank you,