12$ seems a bit high. It would make the article unreadable and any reader would notice that. 12% is definitely not natural and search engines likely think the same way.
I aim for 3%-5%. If you have something like 12% the search engine looks at it as keyword stuffing, and usually marks you as spam.
Just keyword density doesn`t show much. Make some research and you can find that your top 10 competitors have keyword density from 1% up to 25%. Each site/page ranks because of unique score that incorporates complex of ranking factors. If you have a 10 years old site you can increase density even to 20-30% without penalties. Your keyword density can be very high if you have only few words at your homepage but these words are mostly your important keywords. Some words are in some languages used more often than others. Your trustrank matters, history matters etc etc.
I just heard about this at an internet conference from some guys that used to work with google and it is about 4%. Scott
Believe me keyword density was something talked in past !!! Write a natural article with aim in mind to provide your user a valuable info !!