It’s unfair to compare the 3rd generation MiG-21 against the 4th generation F-16. The Western built fighters most similar to the MiG-21 are the Mirage III and the Mirage V. About 2000 built and over 200 still in service. The F-4 is a contemporary but a different sort of fighter. Almost 5200 built, about 300 still in service. (All numbers per Wiki)