How we know when we are using the right number of keywords on a page?

Sadly, I can't let you know a correct number that is the "right" number of keywords on a page, for the most part since that is the wrong approach to consider keyword optimization.

There's no keyword density you ought to go for - indeed, utilizing a keyword too many times can result in penalization due to "keyword stuffing." Just remember the user in mind, and just utilize keywords when you have to. You'll discover enough common chances to incorporate keywords that you won't need to worry about reaching an arbitrary number.

Targeting many keywords from one page is like doing SPAM. It will look like keyword stuffing which will violate Google guidelines. I would suggest target only 3 to 5 keyword from one page and try to use unique and related words of your keywords and avoid using keyword stuffing in the content.