The number of people you ask depends on two main factors:
· The importance of the decision
· The extent to which analysis by different sub groups is needed
The importance of the decision
The more important a decision is, the larger the sample should be that is contacted to take part in a survey.
You may not want to make an important business decision based on 100 people. The larger the sample size, the more confidence you can place on the results. This is why political polling, which gets lots of publicity, is usually based on sample sizes of 2,000+, sometimes substantially more.
The extent to which analysis by different sub groups is needed
Sometimes it is necessary to analyse the results by sub group. When you want to know if there is a difference between women and men, it will be important to ask more people than if you were only interested in the overall findings.