If you are going to put in your title than 2-5 keywords enough if you want to do keyword stuffing it's other thing. you can as many as keywords which you want but in title tag using of so much keyword not good in term of SEO. Make you keyword readable and informative for your visitors not for the search engine.
Used keyword at 2-3 time in title but if you used your keywords many time in title Google will banned your site And mark in a spam.
According to my opinion use single keyword is the best as well as we can use 2 and 3 words titles, title length 65 to 67 is the best lenght of keyword..
On page factors include good Keywords,title,and content of the page. You just make sure that your title should cover all the targeted keywords. And title should be under 70 characters.
Use 2 or 3% of your of full content as keywords, and do not use much keywords in Title the title length should be not more than 60 characters and you can add you two keywords in Title.
You should use one main keyword in title,avoid to use many keywords in title tag it is not good for seo.
Normally the keywords in the title should reflect how it would be like if someone was reading it naturally, so having too many keywords to the point that it doesn't read naturally is not recommended
not more than 3 to 5 keywords. important keywords should be within 65 characters. Dont repeat the keywords Social Networking
refer to this link http://www.seomofo.com/snippet-optimizer.html Type your title and you can see how many characters it uses. You can see live how it looks in search engines. Let me know how it goes. Its depends on characters not words
well actually 2 or 3 keywords are good in title because if you use many keywords then google will also spam it and your site's traffic is going down so avoid putting too many keywords in title tag. it is good according seo and Google
As far as title goes try no to stuff. You want the title to be clearly understandable for a human reader not a spider.
I will suggest using max 3 to 4 keywords in your title. It also depends on the characters of your keywords, for instance if your keywords are short like "weight loss" you can easily use 3 or 4 times. while if they are big like " how to lose weight" it will not be possible to use 4 keywords. Select your keywords smartly and don't change it often it will hurt your rankings.