If you want to generalise from the people you ask (sample) to the whole department/university, you need a large enough sample.
Work it out here: (link)
However, it's tricky - about 250 people are needed to properly talk about a 1000 person department (with 95% confidence that your result is not an accident, and is within 5% of real answer)
However, for practice research projects (i.e. unless you're actually doing your rombun), just state in your Discussion "Weaknesses" section that low sample size was a problem.