Learn Hardcore Indonesian: Polisi mengawasi jalan.

Indonesian normally does not use articles the way English does. So polisi can mean the police, a police officer, or police officers, depending on context. Likewise, jalan can mean the road, a road, or the street.

So Polisi mengawasi jalan could be understood in English as:

The police are monitoring the road
A police officer is watching the street
Police are watching the road

Context tells you which one is most natural.

Does polisi mean one police officer or the police in general?

It can mean either. Indonesian nouns usually do not change form for singular vs. plural.

So polisi may refer to:

a police officer
the police
police officers

If the speaker wants to make plurality very clear, they might use context, a number, or sometimes reduplication in some cases. But in a sentence like this, polisi is naturally ambiguous.

Why is the verb mengawasi and not just awas?