Call (or e-mail) more than you need. A rule of thumb: half the people you contact won’t return your calls. Dealing with that rejection furnishes you with the psychological armor you’ll need during your studies and in your career. Don’t spend ALL your time trying to reach one source. If someone doesn’t respond immediately, try someone else. Don’t overcommit to just one person, even if they seem to be the key to the story.
Different types of stories require different numbers of stories. There are exceptions, but controversial or disputed facts must be confirmed by two sources independently of each other. Official facts — the details of a fire, or a traffic accident, or a campus policy change — can be handled by one source. For stories involving anecdotes — did the Olympics increase foot traffic to local bars? — three colorful sources should be enough to give you a general sense of what the answer is. Keep in mind your deadline, too.