The guys are right. All SEO courses advise webmasters to choose one principal keyword and a few secondary ones.
First of all focus on one keyword, if you are starting to see an increase of traffic keep on going till the keyword is ranked. After, you can start your other keywords, remmeber they should be relevant to your content.
I usualy Concentrate on 3-9 more then 9 it's a bit complicated I worked on 15 but i know SEO .... For an noob amateur is no good ..
I usually focus on 2-3 main keyterms and 8-10 secondary ones. Those long tail SERP's are just as good.
I targeted 30+ keywords in my site's home page along time ago... and it was in good position most of the keywords.
You should concentrate on the number of keywords that your workload can handle. If you are just beginning, start with a single primary keyword. Identify related keywords and synonyms for secondary keywords. These secondary keywords help build up your primary keyword. Do your research and really spend some time finding out how to properly do SEO for your site. From there, add more primary keywords, or turn some of your secondary keywords into additional primary keywords, if you feel you can handle the increased workload. Use the number of keywords that you can realistically continually produce the content for. Take it slow because you don't want to get in over your head and burn out.
One to three at most related keywords (ie blue widget review, blue widget reviews) per PAGE is a good place to start, but you can have more per SITE, so if you have a good ones such as: blue widget reviews buy blue widget blue widget discount Make three separate pages for those, and those pages could have one or two similar variations like 'blue widget discount / blue widget discounts' - optimize the URL of the page, title tags etc for what you are targeting. One of the big reasons for having more than just 1 on the page is so that when you create backlinks, your anchor text is not always exactly the same keyword
It is depends on what types site you have. But from my experience three or four keyword is the better for any site. If you want to use more than that then you can.