The United States conducted around 1,054 nuclear tests
The Soviet Union conducted 715 nuclear tests
The United Kingdom has conducted 45 tests
France conducted 210 nuclear tests
The People's Republic of China conducted 45 tests
The official number of Indian nuclear tests is 6
Pakistan conducted 6 official tests, and around 24 nuclear cold tests
There may have been several other alleged tests.
The increase in background radiation due to these tests peaked in 1963 at about 0.15 mSv per year worldwide, or about 7% of average background dose from all sources.