"once" is an interesting word to use, as many nuclear bombs were set off in space, and many, many more at a high altitude in our atmosphere.
I am sure that none of the atmosphere bombs spread radiation around that people breathed in or consume, nor the underground bombs touched any aquifers.../extreme sarcasm
6 100-mile space detonations counting 5 <2kt fission bombs.
As many as 18, if you count all detonations above ~10 miles.