Use only 2 related keywords in title tag. The meta title must be short and simple. Whatever keyword you select for meta title please note that the keyword must be used in content of the page otherwise it will won't rank.
It is not any compulsion that we have to use particular specific no.of keywords in the title, it depends on our way of submission but i guess 2 or 3 would be enough as title tag should be of 50 to 60 characters.