I have 6 month validity Schengen visa. I want to in how many countries I can travel?
Look carefully at what is written in the "VALID FOR" area in your visa. This is the top line on your Schengen visa. It can say one of the following:
"SCHENGEN STATES" or "SCHENGEN STATEN" or "SCHENGENER STAATEN" (thanks @burhan-khalid and @phoog) - this means you can visit all 26 Schengen countries. Note that you still cannot spend more time in any other country than in the country which issued you a visa for (you'd need to apply for visa for the country you primarily visit).
a specific country/countries name. In this case you legally can only visit the country/countries listed. While there are no border/passport checks between Schengen countries and you might be able to sneak in, this is not recommended. If there is any trouble or random police checks, and you're caught up and have your papers checked, you might be banned from future visits into the whole Schengen area. This would be a quite rare case, but I've seen a visa like that, so it certainly possible.
You can travel to all the 26 countries included in the visa. They can also be found here.