How many words are in a manga?

That entire depends on the series and specific volume or chapter. It's a bit like asking how many Words are in a novel; it varies, often by a great amount. A dialogue-heavy series like Death Note is quite dense, text-wise, and is substantially more Wordy than a series that tells the story more through action, for example Dragon Ball, and though I myself have not encountered any, there may well be series that tell the story exclusively through sequential art with no text at all.