O.k. you got me. 500 is still a lot.
Most of those 500 were fission weapons not fusion bomb in the megaton range. An average of ten a year or one every month is hardly comparable to 40 weapons going off in, say a 72 hour period. If 15 such weapons hit cities in the US you are looking at 15 million fatalities, many more injured or sickened by radiation, transportation, medical facilities, food supplies seriously disrupted or destroyed.
No, you were correct. The USA has tested over 1,000 nukes. Russia, almost as many. Not sure where that poster got their data.