What is the reasonable number of sites hosted in a basic reseller account, to run smoothly. All will be static webcontents, like SMEnterprises, 5 to 10 pages getting approximate visitors : 100, all html / php. In short : basic sites. For example 5 GB disk / 50 gb bdwth, (host like hostgator), reseller account.
If they were all HTML - lots...and lots....and lots as all the server does is pass the pages onto the visitor. HTML pages don't consume many resources. As soon as you bring a scripting or development language such as PHP into the mix it is no longer a "basic" site. Poorly written applications can cause serious problems so it could be as little as 1 web site. There is no hard-and-fast answer, and it depends upon what you want to do.
First I would like to tell you, there are many good web server hosting providers better than hostgator. Don't just look at price, if you want good support and service then choose a reliable service provider. Number of sites hosted on a basic reseller hosting account depends on their data size and traffic.