Generally speaking, 5 reps or less builds strength. We're talking about 85-90% 1 rep max; 10 reps or less best for hypertrophy. More than that is okay for hypertrophy too, but not optimal; you're building more muscular endurance. Perhaps doing >10 might be useful for longer rides...but probably not.

Endurance athletes probably could do well incorporating some kind of weight lifting into their overall training program as the hours of aerobic work likely causes some muscle loss. Lifting weights, especially at the 85-90% max zone, will mitigate some of that loss. Will it make you a better cyclist? No, but it might keep you from looking like this:

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