Tag or tags (or labels in BlogSpot) are the place to put together articles which related to a certain characteristic on a page. It has the function of sorting the article in the specific direction. Tag will narrow the fields for posts that cannot be categorized.
You can use as many tags as you want, even you can insert 1000 tags into the article if you find that is reasonable and useful for your users. However, Tag’s purpose is to connect related articles, so you should select the most relevant tags for your content that users can easily find the article. Do not put too many tags because it will make google think you are spam internal link, so it will not good for users.