The larger the sample size, the smaller the margin of error in the results, but there are also budget limitation, so typically the sample size depends on how the results will be analyzed.
For example, in a survey targeting men and women in their 20s and 30s, there are four demographics: men in their 20s, men in their 30s, women in their 20s, and women in their 30s. Statistically, if there are at least 25 samples per demographic, trends can be observed. Therefore, it would be 25 samples × 4 demographics = 100 sample size.
*There is also a theory that suggests a minimum of 30 samples per demographic.