Hi there, welcome to the Q&A forum!
We get a lot of questions about keyword density here in Q&A As a rule of thumb, I recommend using your primary keyword phrase a minimum of six times on a page -- once each in the title, page description, H1, and alt-image tags, as well as the first paragraph of body copy and URL. Other than that, use the keyword phrase whenever it naturally makes sense to do so. Writing for your audience first and foremost will result in the most natural, effective copy.
This blog post is an excellent primer on keyword targeting: A Visual Guide to Keyword Targeting. And if you search the forum for "keyword density," you'll find many discussions on this topic that look at the topic from various angles, including keyword stuffing.
I hope that helps!
Christy