The precision of your estimates is determined by the sample size. There is a simple rule: an increased sample size will lead to more precise estimates. You can use the tool below to compute the minimal sample size to obtain a required precision.
The first step is to select the confidence level. You can choose between 95% or 99%.
Note that a larger confidence level will reduce the risk of drawing a wrong conclusion, but will also produce wider confidence intervals. So you have two options: either you draw less precisie conclusions with high conficence, or you more precise conclusion with less confindence.