Learn Danish: Flere og flere mennesker cykler til arbejde.

You use flere with countable nouns like mennesker (people). You use mere with uncountable nouns like coffee or time. Since people can be counted, flere is the correct choice here.

Could I use folk instead of mennesker in this sentence?

Yes, Flere og flere folk is also completely natural. Both words mean people. Folk often refers to a general group or crowd, while mennesker focuses on human beings as individuals, but in everyday sentences like this they are interchangeable.

Why does the sentence say til arbejde without an article instead of til arbejdet?

Danish often drops the definite article in fixed phrases about daily routines. Just like in English where we say to work rather than to the work, Danish uses til arbejde when referring to the general activity of working instead of a specific physical workplace.

How do I know if cykler means cycle or are cycling?

Danish does not have a continuous tense like the English -ing form. The present tense verb cykler covers both general habits and ongoing actions. You just rely on the context of the sentence to know which meaning fits best.