The standard deviation is:
The confidence interval is 95%
You want to be confident that your estimate is within 10 of the true population mean, then you can use the next formula:
Where ME is the marginal error, z is the z-score (for a confidence interval of 95% is 1.96), s is the standard deviation and n the sample size.
Then, by replacing the values:
Thus, you need a sample size of at least 204 people.