I had five (total) letters the first time I applied: my history/humanities prof, my gen chem/ochem prof, my physiology prof, my genetics prof, and my volunteer leader. I decided after the cycle was over that my genetics and chem professors probably didn't know me well enough to write an amazing letter for me.
So the second time I applied, I had: my history/humanities prof (the same letter, as I couldn't get in touch with him to update it), my physiology prof (rewritten letter, since I took my senior seminar with him as well), my biochem prof, and my supervisor at work. I got four interviews (of 9 schools) and 2 acceptances.
If you can't get an updated letter, and you're sure that the letter was good, there's no harm in reusing it.