Yes, lunges provide one of the most effective lower-body strength-training exercises out there -- but that doesn't mean you should be doing 50 at a time. For most healthy adults, a single set of eight to 12 repetitions per leg suffices, adding weights as needed to increase the challenge. Older adults, or those more interested in developing muscular endurance as opposed to strength, may be more comfortable doing 10 to 15 repetitions with less or no added resistance.
A Few Exceptions
If you're focusing purely on developing strength or if you're bodybuilding, you'll follow a slightly different training regimen -- but the number of lunges you do will go down, not up. For building strength, the American Council on Exercise recommends doing two to six sets of four to eight repetitions each, with two to five minutes of rest between each set.
For bodybuilders, ACE recommends three to six sets of six to 12 repetitions each, with 30 to 90 seconds of rest between each set.
Hitting Your Target Number
If you can't complete your minimum target number of lunges, carry less weight or -- if you're not using weights -- switch to an easier exercise. If your leg muscles aren't tired by the time you finish your set, it's time to carry extra weight or switch to a more challenging type of lunge.