How many keywords we can put in a website and how many characters should be in a keyword ?
You can put in many keywords. Their character lengths will vary.
But keywords put aside for a moment. Why not simply sprucen up on your language skills and learn the art of writing sound SEO copy? This way when you become skilled at it you'll be crafting useful, unique things for the Internet which will naturally rank well in a variety of keywords.
Use as many keywords you want to use in the webstie and there is no limit for the charcters you use in the keywords . Just make shure that the meta title is no longer than 65 words with spaces .