The more fingers you add to Touch ID, the less selective it will be. For example, if you train the system with four fingers, then it will be four times as likely to match on a stranger's finger.
From Apple's iOS Security Security Guide, p. 10 (edited):
The probability that a random person in the population could unlock your iPhone is 1 in 50,000 with Touch ID.... This probability increases with multiple enrolled fingerprints (up to 1 in 10,000 with five fingerprints)....
So, you should train the fingers you need for the system to be convenient, but no more, and remember that you're balancing convenience against security.