Probably the same place 'realities' or 'universes' came from. Nouns have plurals even if the actual plurality makes no sense

I think they were using phrases like "burn you with the heat of a thousand suns!" Long before they realized there actually were more suns out there.

The year was 1968. The National Basketball Association was expanding and two cities were selected. One choice would go to Milwaukee and that team would choose the Buck as its symbol) nickname. The other choice would be Phoenix, in the Valley of the Sun and need to be the first representation of professional sports in the entire state of Arizona. As the Sun was so important to the identity of Phoenix, they would also be the Sun. But what would the group be if each player was a Sun?

In the morning I greet a bleary, foggy eyed sun. At noon, I see a cruel and imposing sun. In the evening I see a sun, red and weeping into the sea. At night, where have my suns gone?

historically many early humans thought the sun was a different sun every day.

Tatooine maybe?

Well, you see...

My take is that the word "suns" would imply someone else's Sun in various systems. I don't think it would be correct to call a star a sun if it didn't have planets and occupants (past or future).

Since "the" is needed, "the Sun" is more of a generic placeholder, whereas Sol is its actual name in English. So you could be on a different planet somewhere and refer to its star as the Sun (implied of this system), or its name.

It's also quite possible "suns" has been used in the past for all stars, this is just my take given how the words tend to be used by us.

In what context did you hear the word "suns"? Presumably, it had to be invented for whatever context required the plural of sun. In Star Wars, Tatooine has two suns. In the galaxy, there are many suns. It's not like it took much thinking to figure out how to pluralize sun.

Also, in ancient times I think "suns" was another way to say "days"