To answer your question, there are 4 semiquavers in a crotchet and 2 crotchets in a minim. Since a dot indicates a note half as long again as its written value, then a dotted minim = 3 crotchets = 12 semiquavers.
However, if you want to get serious as a musician, then visit kempie's link and study it well and learn it until you know it without thinking about it.
Thank you both so much for your help. I have been on the sites suggested by kempie, which were excellent, and they made me realise how much I'd forgotten. Even better, I also found the answer to another music question which I needed for the quiz I'm doing. Many thanks again.