How many countries the world have?

Answer 1

No country has ruled the entire world. However, there have been empires that have ruled the 'known' world from their perspective.

Perhaps the first great global ruler was Alexander of Macedonia (Alexander the great)

Then there was the Roman Empire

The Holey Roman Empire

and perhaps the largest of all, The British Empire.

In modern times counties/empires seem to be fragmenting rather than growing.