The ideal size of an NPS survey sample set depends on the following factors:
The margin of error with which you are comfortable
Your desired standard deviation
The likely distribution of promoters, passive, and detractors in your sample set
If you’re comfortable with standard deviations and margins of error, you can use statistical analysis to calculate the ideal size of your sample set. If you’re terrified, follow this rule instead: Assume your sample set is representative of the whole. So, the more customers you survey, the more accurate your data will be.