Minimum Sample Size required

#1
Hi everyone, I was wondering if you can help me track down an answer I should probably already know how to find. Assuming I take 1000 calls in a month, what is the minimum number of surveys from customers I would need to get for a 95% confidence that it accurately reflects the service I'm giving? And/or what would be the formula so that I could adjust it for months I only get 600 calls?