Learn Turkish: Polis, birtakım isimsiz ihbarlar aldı. - The police received a number of anonymous tip-offs.

You are exactly right that words like birkaç (a few) or birçok (a lot) must take singular nouns. However, birtakım (meaning 'a number of' or 'certain') is a special exception to this rule. It always takes a plural noun, which is why we see ihbarlar here.

Is birtakım always written as a single word?

Yes, when it is used as a vague quantifier meaning 'a number of' or 'certain', it is always written together. If you write it as two separate words (bir takım), it literally means 'one team' or 'one set', such as in the phrase bir takım elbise (a suit).

How exactly does isimsiz mean 'anonymous'?

It is formed from the root noun isim (name) and the privative suffix -siz (without). It literally translates to 'nameless' or 'without a name', which is how Turkish expresses the concept of an anonymous person or thing.

Why is there a comma directly after Polis?

In Turkish, commas are frequently used right after the subject to clearly separate it from the rest of the sentence. This ensures you know Polis is the one doing the receiving and prevents you from accidentally misreading it as part of a compound noun phrase with the words that follow it.