How many customers can be handled on a single server?

The number of connections a single server can handle mainly depends on the configuration of RAM and CPU. More the space and capacity of the hardware, more the users a server can handle.
The server’s availability also depends on the type of transaction a user is going to make on a website. Say, how long the user has opened a shopping web page or making a financial transaction.
One more factor to consider is the scope of the website. Certain website attracts many users at a time. For example, on a page that has registration details for a scheduled marathon, many users tend to register at the last minute. In that case, the website will be unavailable or inevitably slow!
To summarize, there is no such final number on the number of connections that a single server can handle. It is mostly determined by the configuration of the server and the expected traffic.