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.
Firstly, we need to figure out each site's files, network traffic each site consumes, CPU resources requirements, Server Configuration. Once, you find out the exact data for these, then you can know how many sites will fit on one server. Also, the world's most popular sites, such as Google, YouTube, Facebook, etc require many servers to run just single site from the user's point of view.
That's not a great question, as it does not address any possible variables (which is basically the only constant in web hosting).
Host #1 may answer 100 and Host #2 answers 50. Host #2 only fills servers half as much as Host #1, so therefore, it is exactly two times better (50x2=100..duh). Honestly, this is the type of thing hosts don't want to share as it is a private matter between the host and their license distributor. Also, some hosts allow users to have astonishing resource allocations, thereby allowing them to squeeze tons of sites into one account, which is dumb, but I think the industry term is "Unlimited Hosting".
If you want to know how crowded hosting servers are, then a better question would be "What server hardware are you building your cPanel nodes on, or are you running cPanel from hypervisors? Another great question: "How are you managing different server components in your cPanel environment, or just all deployed and managed from central server?" This question will if nothing else, provide insight on the varying hardware makeup of cPanel servers across different providers.
If you are more interested in knowing how costly each hosting provider's licenses will become (due to the recent pricing model adjustments), then variables such as whether the host is a cPanel partner and if so, what tier of discounted licenses are they receiving. Also, the original question suggests that all Accounts are equal, which is not often the case. In fact, if you're hosting a server of real, active clients/users, you know that they are all unique. The composition, resource usage, and size can change rapidly.
Let's discuss something of substance, though...Your choice, but I will play along, to the best of my ability.