Depends on the book. ~ 250 words was the standard a long time ago, before they realized that it didn't work for everything.
Why don't you count them? It's not hard. I've done so many times. (Or you can estimate by counting the number of words on ten lines, divide by ten to get the average number of words per line, then multiply that by the number of lines on the page.) In most of the paperback books I've checked, it's about 400 to 430 words per full page. Some larger-format books, like trade paperbacks, have larger print, so it's closer to 250 to 300 words per full page. But, come on, pull a book out of your shelves and count the words! The answer is right there in your hands!
It varies by page size and font size, and it is completely unimportant. Publishers only care about total word counts, not page counts.
