I'm not entirely sure of your design but I believe you can find what you need at this link:

https://www.google.com/search?q=sample+size+calculation+for+comparative+cross+sectional+studies&ie=utf-8&oe=utf-8&client=firefox-b-1

Best wishes, David Booth

I'm looking for the answer for this question too.

Hi Aboo,

How are you doing ?

Fine, I hope.

Regarding your question, I don’t know a specific formula for that but when I work w/ SEM I use a rule of thumb of 5 - 10 respondents by each item in my questionnarie (I have a paper related to this if you want).

Hope this help.

Regards.