That depends entirely on what your code is doing. Free Hosting accounts have unmetered bandwidth, but there is a limit on the memory (64MB combined from all concurrent processes) and processor cycles (300 CPU-seconds per hour) you can use. A site that is made up only of static files (HTML, CSS, a few small images and maybe some JavaScript, with no PHP or database calls) can handle a lot of traffic; heavy-duty CMS-based sites, Facebook clones, collaboration systems and Student Information Systems (especially) can reach the resource limits with very few users. Some software packages require a lot of resources, and can't even manage a single user. (The same is, unfortunately, true of a lot of beginner PHP code that works, but does things in very inefficient ways.)