Notion is packed with features that, frankly, can go unnoticed. One simple, elegant feature is the 'word count' tally, which is present on every single Notion page: so long as you know where to look for it.
Unfortunately, unlike other common word processors, Notion does not at this time let you get a quick word count on a selected portion of text. You'll only be able to get a value for the full page, not just a section of that page.
Another common way you might want to use word count is as a 'property' in a table. So that if you were writing blog posts, say in a content database, you could automatically rollup that word count value from the page itself and display in a column of your table.
Sorry to be the bearer of bad news once more, but as of this moment Notion doesn't let you simply take the word count value as a database property (or at least, not in any way that I'm aware of).
You can of course still achieve the same end result by checking the word count value for each page, following the steps shown above, and listing that value in a property 'Word count' in your database.
If, on the other hand, you were looking for a way to count the words within another database property (and not the page itself), you can take a look at this Medium article for a formula that is supposed to do the trick.
That's it. And if you're interested in more handy Notion features, why not learn about this simple Notion Database template hack?