Think you know the answer? Well I was a little surprised to find that it is over 2000 according to this piece over at ZeroHedge :

Who needs a wartime nuclear exchange when you have peaceful countries nuking the gamma rays out of their own sovereign territories – now that the environmental theme is rather popular, the following video by Isao Hashimoto shows all the nuclear “tests” conducted by the world in the period between 1945 and 1998. Based on public data, the world’s peaceful countries have already nuked themselves at least 2,054 times, with the US nuking the state of Nevada and its immediate neighbors about one thousand times. And keep in mind – the fallout does not just miraculously “disappear.” Feel free to consider that next time you look at bargain properties on the strip. Anyway, as Idealist.ws asks, “How would your life be different if you were taught in school a small nuclear war already took place?“

And, if you want to know places to avoid when choosing your next holiday, the map below gives full details of all the known tests (click image for larger view) :