You are correct that the noun "police" is treated as a plural.

Uncountable?

It isn't an uncountable or mass noun. If it were, we could say things like "not much police" and "only a little police", like we can with "salt", "mud", "rice", "rain", etc. Instead, we say things like "many police" and "few police", like with any other plural noun.

Plural

Longman's A Comprehensive Grammar of the English Language (Quirk et al., 1985) refers to "police" and "people" as "unmarked plural nouns".

The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) refers to "police" as an "uninflected plural-only noun". Such words are said to be "quasi-count nouns" because they cannot be used with low-number numerals. Hence, we can't say "four police", whereas we can say "two hundred police".

"Polices"

You can't say "polices". You could say "types of police", "kinds of police", or "police forces" (but the regular or civilian police may itself consist of multiple forces, e.g. different forces for different localities, and so "types of police" may be clearer).

You are correct that the noun "police" is treated as a plural.

Uncountable?

It isn't a typical mass noun. If it were, we could say things like "not much police" and "only a little police", like we can with "salt", "mud", "rice", "rain", etc. Instead, we say things like "many police" and "few police", like with any other plural noun.

Plural

Longman's A Comprehensive Grammar of the English Language (Quirk et al., 1985) refers to "police" and "people" as "unmarked plural nouns".

The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) refers to "police" as an "uninflected plural-only noun". Such words are said to be "quasi-count nouns" because they cannot be used with low-number numerals. Hence, we can't say "four police", whereas we can say "two hundred police".

"Polices"

To refer to multiple police forces, you could say "types of police", "kinds of police", or "police forces" (but the regular or civilian police may itself consist of multiple forces, e.g. different forces for different localities, and so "types of police" may be clearer).

In at least some varieties of English, we sometimes see "polices" used to mean multiple police forces, as in "The polices of various states are cooperating".

You are correct that the noun "police" is treated as a plural.

(It isn't an uncountable or mass noun, as has been incorrectly stated by some. If it were, we could say things like "not much police" and "only a little police", like we can with "salt", "mud", "rice", "rain", etc. Instead, we say things like "many police" and "few police", like with any other plural noun.)

You can't say "polices". You could say "types of police", "kinds of police", or "police forces" (but the regular or civilian police may itself consist of multiple forces, e.g. different forces for different localities, and so "types of police" may be clearer).

You are correct that the noun "police" is treated as a plural.

Uncountable?

It isn't an uncountable or mass noun. If it were, we could say things like "not much police" and "only a little police", like we can with "salt", "mud", "rice", "rain", etc. Instead, we say things like "many police" and "few police", like with any other plural noun.

Plural

Longman's A Comprehensive Grammar of the English Language (Quirk et al., 1985) refers to "police" and "people" as "unmarked plural nouns".

The Cambridge Grammar of the English Language (Huddleston & Pullum, 2002) refers to "police" as an "uninflected plural-only noun". Such words are said to be "quasi-count nouns" because they cannot be used with low-number numerals. Hence, we can't say "four police", whereas we can say "two hundred police".

"Polices"

You can't say "polices". You could say "types of police", "kinds of police", or "police forces" (but the regular or civilian police may itself consist of multiple forces, e.g. different forces for different localities, and so "types of police" may be clearer).