There is no way to determine the maximum number of visitors your site can sustain. Each site is different and the amount of traffic you can handle will depend on the resources each visitor is using.
Once you are online, you can view your site statistics, like the number of visitors and which browser they use. Your cPanel includes a statistics program called AWStats.
If I need to handle more traffic, what should I do?
Shared, reseller, and VPS all handle the same amount of traffic. My best recommendation is to upgrade to a dedicated server, or a more powerful dedicated server.