statistics - sample size?

I am writing a thesis on "Violence in NHS Hospitals". Part of my investigation involves a study on NHS Staff. I have a questionnaire with 15 questions to apply but cannot calculate the sample size to make this research project representative to the population. Cuantitative variables have a formula to calculate this after a pilot test, but my main variable is CUALITATIVE, how do I calculate it? HEEEELLP!

Forget about it it doesn't happen say NHS staff

I don't know how you calculate the sample size but spending any time in an NHS A&E department would show that a large proportion of staff are abused physically and verbally .