The Mig-21 iwas the most prolific jet fighter ever built.
Even without considering the Chinese J-7, there were 11,500 Mig-21s built. When you add in the 2,400+ J-7s, you end up with almost 14,000 of them.
The most prolific US/Western fighter was the F-86 Sabre, of which almost 9,900 were built.
By comparison, the F-4 Phantom, which came along a little later, was built to 5,400 aircraft.
The F-16 has so far built about 4,600...but they are not done yet and may yet exceed the F-4.
But I do not believe any new. modern fighter of any sort will ever be built in the numbers of the Mig-21.