Learn Hardcore Greek: Ένα εκατομμύριο είναι μεγάλος αριθμός.

Because εκατομμύριο is a neuter singular noun in Greek. The word ένας/μία/ένα has to agree with the noun’s gender, so with εκατομμύριο you use ένα.

Does ένα here mean a or one?

Formally, it can be either, because Greek uses the same word form in many contexts. In this sentence, with εκατομμύριο, it is naturally understood as one: one million.

Is εκατομμύριο really a noun in Greek?

Yes. In Greek, εκατομμύριο is basically a neuter noun meaning million. That is why it can take words like ένα before it, and why it has a plural form: εκατομμύρια.

What form is είναι?

Είναι is the present-tense form of είμαι (to be). It can mean both is and are, and the context tells you which one it is. Here it means is.

Why is it μεγάλος αριθμός and not ?