Traditionally Enterprise CRM is associated with large companies and large teams needing a very high level of interdependency. Large teams need to have a process in place which will help companies manage their sales, marketing, inventory, etc. in a smooth fashion. Furthermore, access to the company data is based on the type of task being performed by someone or by someone in a particular rank in the hierarchy.
There are multiple ways by which this question can be approached.
Those companies which require features like Auto-Responders, Data-sharing rules, Groups, unlimited Roles and Profiles, they need to have the Zoho CRM Enterprise Edition.
Typically companies having 800+ employees require complex hierarchical control to restrict access to data based on the roles/profiles/hierarchy of the people within the organization.
The more complicated the Sales/Marketing Team hierarchies, the more the need for a system which is flexible and is able to accommodate multiple hierarchies and profiles.
Zoho CRM does not enforce a minimum number of users to sign up to get the Enterprise Edition. We realize that all companies will have their own way of handling their CRM so they can choose what suits them best.