Dear Ayesha, refer to my answers to this question:

https://www.researchgate.net/post/What_is_an_appropriate_sample_size_for_an_infinite_population

Thankyou Ehsan for your valuable information.

I assume Its researcher's discretion depending upon the kind of sample and study.

Hi Ayesha,

One of the classic articles showing how to determine sample size is:

Krejcie, R. V., & Morgan, D. W. (1970). Determining Sample Size for Research Activities. Educational and Psychological Measurement, 1970(30), 607-610.

Their pdf article can be downloaded from this link:

http://opa.uprrp.edu/opaweb/InvInsDocs/KrejcieandMorgan.pdf

There are also many online sample size calculator from Internet search, one of them as below:

http://www.calculator.net/sample-size-calculator.html

Regards,

Fung