How do I know when I’m using the right number of keywords on a page?
 

Please explain in details .
Thank You

If your Page started already ranking into SERPs (Search Engine Results Pages) of Search Engines which you are targeting, then what you are doing is already correct, you’ve already used right number of Keywords on a Page.

Different SEOer’s will have different opinions, regarding Keywords density, LSI Keywords, Meta Tags, etc. But it’s you who knows what is working for you. Stick to it and continue the same to other Webpages of your Website.

However, if your Webpages is not yet ranking in SERPs, you should check if all the SEO elements are in proper shape or not.

The most easiest & free tool to audit your Webpage is SEOQuake.