For most people, a single set of 12 repetitions with the proper weight can build strength and improve fitness as effectively as can multiple sets of the same exercise.
The one-set approach also has the advantage of saving time, which makes it easier to fit into an exercise routine. Simply choose a weight that tires your muscles around the 12th repetition. As this becomes easier, gradually increase the amount of weight to maintain a 12-repetition fatigue.
There's a caveat, though: If you're a bodybuilder or an elite athlete with specific performance enhancement goals, additional strength training sets may be beneficial.