"Police officers are coming."
Translation:Des policiers arrivent.
I agree this is a little confusing - venir is usually a directive, as you would say "come here" to someone. Or you use venir to describe where in the world you are from. But the french always use arriver when it relates to time. So if you think of this English phrase as "Police officers are coming soon" then you would say "des policiers arrivent bientôt"
"Des" is the plural of "un" or "une" and it is required to mean "more than one".
Police officers are coming =des policiers arrivent
the police officers are coming = les policiers arrivent