How many countries did the stock market effect?

Practically all Western countries, including those of South America were directly affected. Not so much because of Stock Market losses (outside of the US, playing the market was a game for only the more adventurously minded elite) but the resulting economic crisis led many countries to adopt protective measures for their domestic markets. It apparently dawned on nobody at the time that if every country did this, worldwide trade would more or less come to a standstill. And so it did, which caused often massive unemployment in all the countries concerned, which caused an enormous collapse in wage income and therefore, in customer demand; which in turn led to bankruptcies of shops and of the small to medium-sized businesses that supplied them; which led to even more unemployment, and so on.

Third World countries usually escaped the consequences of the trade collapse. Most of their exports went to their mother country and trade with the colonies usually fell under the 'protected home trade'.