There is no answer to this question. It depends on the server and the type of customers and websites hosted on that server. But one machine can host hundreds of websites even thousands. Most customers worry about how much resources they will have with grand desires to have a super busy website and lots of visitors. But that is not usually the case. Most often than not most websites only use a very small fraction of what is allocated to the account. So when factoring in how many websites a machine can host you need to consider that most wont use all their resources and a very small faction will use more than what they have. But then there is WordPress which can use a significant amount of RAM and IOPS. So this needs to be considered also the type of website hosted.