Two addresses, a primary and a secondary (back-up). But your system is likely set to obtain DNS server automatically.
As most of you probably know, IP addresses are actually made up of 4 nodes of numbers seperated by ".", where each node can be from 0 to 255. Most people can't remember these numbers so we use words (another advantage is that a site can be moved to a different computer with a new IP address but keep the same "words"). A DNS servers purpose is to look up the words and convert it to the number. The longer it takes for this conversion, the longer it takes the browser to get to the site. This can vary by time of day because of higher processing volumes during the workday, "hops" between you and the DNS server, etc.
Also DNS servers may not be public; you can use YOUR ISPs server but most people in the world probably can't. Also some DNS servers take bad addresses and automatically redirect you to there search engines etc instead of returning a 404 Not Found, and you may not like this.
So, if you want to change your DNS server, compare your own and several Open sites at various times of the day. Once you have your choice to try, here is how to change your settings...
http://mintywhite.com/windows-7/chan...+%26+Guides%29
Note: In step 3 it may be Local Area Connection or Wireless Connection depending what you use (maybe even bluetooth if you connect over your phone)
Note2: Before changing anything in Step 5, see if it is set to "obtain addresses automatically" or not, write down this setting and any IP addresses if present (in case it doesn't work as well as hoped).