Basically, if you have a 400-word article, your keyword phrase should be used no more than 4 times or this is considered "overstuffed". Not only can this get you a penalty when posting online but it can also make the article more difficult for the human reader to comprehend. Too many keywords in there will make it read unnatural and the sentences will not flow.
I think it depends. I always aim for 2% keywords density, though. For the tags, I prefer to put about 5-10 tags in a blog post. I don't like to put more keywords than that because I often found out that the article looks "strange".
This is what you can say Keyword stuffing, Means there should be only 3-5% of total keywords in a single page of article with compared to total no of words in that particular page. More than that may cause Spamming
I suggest to put 2 keywords in a article and it can be used 1 to 2 times in 100 words. So if a article with 400 words then the keyword(s) must be kept 4 to 8 times in that article.
I agree with others that 5 to 8 keywords in the article works best as long as it appears naturally in the content and not just thrown in for SEO purposes. As for tagging, I would not worry about the number as long as they are relevant to your article. What also helps is if you can backlink one the keywords to an authority site like wikipedia which helps to make your page look relevant in google's eyes. Good Luck.
Dont stuff in the articles too much with the keywords. Use atleast 5-8 keywords in your article, with only two keywords to be alloted with hyperlink. You can put as much as tags you want, make sure they are all relevant to your article. However I would prefer around 10 -12 tags with each tag being a common keyword for the article.