Hello,
How to maintain keyword density?

There is no optimal KEYWORD DENSITY. And the moderately optimized text is going to perform better than repeating keywords too often. If you read a page and find you are repeating the keyword phrase too often, you are probably keyword stuffing your text.

The keyword density score for a page is essentially calculated looking at how many times a specific key phrase is repeated in a document, the number of words in that key phrase and the total number of words in the analysed text.