If you have to ask that question, I would say you are using too many.
The on page tool looks for one instance of a keyword in either a strong, bold or em tag so if you are doing this for SEO, the prevailing common sense would be that any more than that one is too many.
If the highlighting provides some kind of benefit for your users then you can do more but if you are looking at bolding lots of text for SEO - then forget it.
The best answer is to just use common sense and create something that is readable with the important keyword highlighted if you can do so in a natural way.
Hope it helps.
Marcus