There are several methods could be use to determine the sample size of the study:
1- Krejcie and Morgan table see :(Krejcie, R. V., & Morgan, D. W. (1970). Determining sample size for research activities. Educational and psychological measurement, 30(3), 607-610).
2- Range Rule of Thumb 30 to 500
3- Use previous studies results(mean and standard deviation)
4- Use raosoft website -http://www.raosoft.com/samplesize.html
So, u can either consider ur population or previous related studies to calculate ur sample size.
There are several methods could be use to determine the sample size of the study:
1- Krejcie and Morgan table see :(Krejcie, R. V., & Morgan, D. W. (1970). Determining sample size for research activities. Educational and psychological measurement, 30(3), 607-610).
2- Range Rule of Thumb 30 to 500
3- Use previous studies results(mean and standard deviation)
4- Use raosoft website -http://www.raosoft.com/samplesize.html
So, u can either consider ur population or previous related studies to calculate ur sample size.
Since you have subcategories, first divide the target population into subgroups and select the sample size based on proportion they occur in the population. The sample size formula also depends on your research design, so the rule of thumb can be varied. You can use G POWER or a simple sample size calculator in Survey monkey: https://www.surveymonkey.com/mp/sample-size-calculator/