Learn Swedish: Jag ser många människor på gatan. - I see many people on the street.

We use många (many) because människor (people) is a countable noun—you can literally count one person, two people, etc. You use mycket (much / a lot of) for uncountable things, like mycket kaffe (a lot of coffee).

Is människor a plural word? What is the singular form?

Yes, människor means "people" or "humans" and is plural. The singular form is en människa (a person/human). It belongs to a common group of words ending in -a that take an -or ending to become plural.

Why does the word gatan end in an -n?

That -n at the end makes it mean "the street". The basic word is en gata (a street). Because it is an en-word that already ends in a vowel, you just add -n to the end to get the definite "the" form: gatan.

Could I use the verb tittar instead of ser in this sentence?

Usually no, because they mean slightly different things. Ser means "see" (just noticing something with your eyes, like in this sentence), while tittar på means "look at" or "watch" (actively focusing your eyes on something).