Ammon Johns: How many words in how many languages can Google provide a relevant and useful page for? That`s how many keywords Google have then, sort of.
You just asked "How many words can I be relevant for?" but without telling us how many topics you know, how deeply, and in how many articles you provide information on those that is better than the vast majority of pages elsewhere on the web.
In short: You`re asking entirely the wrong question.
Think about this to get a better mindset: How many searches would a page in Amazon (any/all nations) be a relevant and useful result for? Millions if not billions, right?
But despite all those billions of searches on millions of phrases, 80% of their business or more comes from just one single keyword: "Amazon"
When you understand the meaning of that, you`ll be in a far more advanced position.