The tags are intended to describe the specific details of your posts. You have to see them as keywords that appear in a hypothetical index of your website. They are micro-information that can be used for micro-classify your content.
The sole purpose of tags is to classify the content of your WordPress to enhance the usability of your visitors. Through tags your visitors can find related content on a specific topic.
You can add as many tags as you please. However, keep in mind that the purpose of the tags is to link your content with each other. So if you end up having so many tags that each one is only added in a single post, this makes no sense. Try not to put more than ten tags in a post, unless that can be justified.