You can have 1 INDP server supporting 100s of printers. In fact I suspect this will be a common configuration. A spooler will use INDP to get machine readable notifciations from the devices it is using and will then genrate email to various interested parties - who may well have used IPP to ask for email notification. The advantage here is the the spooler can generate consistent, localized messages. There is a great difference in cost, complexity, resource consumption, and security considerations between running a client on the Internet and deploying a server on the Internet.
Most Internet servers are used by many users, so the cost is affordable.
A server per user just won't scale to the Internet. Peter Z. has a suggestion for helping to determine what is supported. However, this ignores the fact that INDP also handles human readable. Following the SF meeting I would like to formally propose the following.