Learn Swedish: Några resande är redan här. - Some travelers are already here.

Resande belongs to a special group of nouns that end in -ande or -ende and describe people. These words do not change at all in the indefinite plural. One traveler is en resande, and some travelers are några resande.

Since resande looks the same in singular and plural, how do we know it means multiple travelers here?

The word Några (some) tells us it is plural. Without a clue like några, flera (several), or a number, the word resande by itself could mean either one traveler or multiple travelers.

Is resande a noun or a verb form in this sentence?

It acts as a noun here, but it comes from the present participle of the verb att resa (to travel), which normally translates to 'traveling'. Swedish frequently turns these -ande verb forms into nouns to describe a person performing that action.

Could I use the word resenärer instead of resande?

Absolutely! En resenär (plural: resenärer) is a very common synonym for a traveler or passenger. Resande focuses a bit more on the ongoing action of traveling, but they both work perfectly in this context.