As reported by the New York Times.
The article reports:
According to one estimate by a physicians group opposed to nuclear weapons, 2.4 million people could die from cancer caused by the radioactivity from the more than 2,000 known tests that have already taken place.
The link above points to the website of the Comprehensive Test Ban Treaty Organization, which reports:
An article featured in Volume 94 of American Scientist on Fallout from Nuclear Weapons Tests and Cancer Risks states that a number of studies of biological samples (including bone, thyroid glands and other tissues) have provided increasing proof that specific radionuclides in fallout are implicated in fallout-related cancers.
It is difficult to assess the number of deaths that might be attributed to radiation exposure from nuclear testing. Some studies and evaluations, including an assessment by Arjun Makhijani on the health effects of nuclear weapon complexes, estimate that cancer fatalities due to the global radiation doses from the atmospheric nuclear testing programmes of the five nuclear-weapon States amount to hundreds of thousands. A 1991 study by the International Physicians for the Prevention of Nuclear War (IPPNW) estimated that the radiation and radioactive materials from atmospheric testing taken in by people up until the year 2000 would cause 430,000 cancer deaths, some of which had already occurred by the time the results were published. The study predicted that roughly 2.4 million people could eventually die from cancer as a result of atmospheric testing.