It used to be the case that you could have a total of 5, regardless of whether they were in the hashtag box or in the form of "#" within the body of the post. As far as I know, this is still the case, but I'm not sure.
As far as SEO purposes go - I'm not an expert in that area and I can't give you the good answer. To the best of my knowledge, the title and the content matter a lot more than any hashtags.
Ok thanks. Hashtags can show trends like on twitter and are used by marketing companies for statistical analysis. If you click a hashtag for example, you see all the recent posts with the same hashtag. Try it 😊