This is entirely up to the web host and, as such, those considering shared hosting should reasearch very carefully which web host they are going to use.
We have years of experience at abdicar.com. We know of cases where some hosting providers will host over 500 domains per shared hosting server.
At abdicar.com, it depends on the hosting package that you select, but we will normally limit the accounts on a server to 30 customers and their domains. As you know, we offer unmetered web hosting solutions.
However, when any customer reaches a considerable amount of domains and needs to use considerable resources, we will motivate him/her to move to a Virtual Private Server.
At abdicar.com, we never oversell so that resources can be available for our customers’ needs.