There is no number of keywords for a blog or a website. It's a number per page as it's the pages that are ranked, not entire sites or blogs. Each page should be about a very specific topic so the number of keywords it targets, intentionally or not, shouldn't be very large. If you find a page covers a large number of keywords that may be a sign it needs to be split up into multiple pages or posts.
I'm not going to give you a specific number of keywords to target per page as not only isn't there a set number, but you should be writing your content for your users and not focusing on how you can fit keywords into the content. If you write your content properly you'll find that approximately a handful of popular/semi-popular keywords will be targeted naturally for you with a bunch of long tail keywords sprinkled in, too.