Simple question: When you're on the course and you have a putt you have to make, how many chances do you get? Exactly. So use that same philosophy when you practice your putting.
Mark the ball, read the putt, and then put your best stroke on it. If you miss, mark the ball, read the putt, and put your best stroke on it again—one golfer, one ball, one putt. The pros practice this way all the time, and it's a big reason why the average player on the PGA Tour has at least seven one-putts per round. (Troy Matteson had 19 putts total in the second round of the Greenbrier Classic in July.)
If you get only one chance to make a putt during a round, it doesn't make sense to drop four balls on the practice green and repeat a putt four times. It's not a good way to simulate performance, and it does little to build confidence. Practice the way you play, and on that next must-make putt, you'll be prepared.
On the practice green, focus on six-footers and 30-footers. Tour pros expect to make it from six feet, and they three-putt as often as they one-putt from roughly 30 feet. Learning how to two-putt the long ones will keep you in the match long enough for you to drain the short ones and win it.