How many user accounts are there on your hosting server? You can look up the number of server user accounts by the following command via SSH:
wc -l /etc/passwd
And it may output something like this:
Which means there are a total of 76 active users in the system. However, the actual number of human users of the server should be lower than the amount because there are system users created to carry out certain tasks.
Multiply the amount by 5 and you may get a rough number of websites hosted on your server.