ISP's usually cram a couple (or a couple hundred) web sites onto a single server. This is possible because the average web site doesn't get much traffic. This makes sense for the ISP because they have less hardware to maintain and less expenses ( licenses, electricity, admin overhead).
If one has a particularly popular site, then one can purchase a dedicated server, which means one's web site is the only one hosted on a particular physical server. This is, of course, significantly more expensive than a shared server.