Sample Size and Margin of Error Problem


It would be great to get your help on a stats problem I have as I am very new to the field.

Say I conduct a survey which aims to see how much people who are part of a social benefits scheme like the scheme. The total number of recipients of the scheme is 250 yet I can get 96 people to fill in an answer.

The answers are:
Yes, like the scheme 30%
No, don't like the scheme 70%

My question is, how do I know if my sample size is big enough at a particular confidence interval. Is it a case of finding a 'margin of error' based on the population and sample size. For example, since I can only get 96 people to fill out the form, at a confidence interval of 95% I can get a margin of error of +-9 (for example).

Many thanks for your help