WordPress lets you put a post into multiple categories. This could be several parent categories or a parent category plus a subcategory or subcategories.
Having multiple categories won’t benefit your SEO. You should only assign posts to multiple categories if it makes the most sense for your readers.
It’s possible that having your post in multiple categories could cause some SEO issues due to duplicate content.
If you do use multiple categories, then try to avoid putting one post into two or more main (parent) categories. Each post should fit within one main category.