QuickBooks is a multi-user program, as opposed to a "true" network program. Network programs are typically installed on a single server or workstation, and then accessed from that installation by multiple users simultaneously. In contrast, every QuickBooks user on the network must have a separate, licensed copy of QuickBooks installed on their local workstations.
However, while the QuickBooks program must be installed on each workstation, the company data file can be stored on a single server or workstation and accessed by multiple users. QuickBooks allows up to five users (15 users for QuickBooks Enterprise) simultaneous access to the data file across the network.
Note: QuickBooks Enterprise 7.0 and above allows 20 users.
You can learn about Setting up a network or multi-user environment.
The only exception to the above is if QuickBooks is deployed on your network in a Microsoft Terminal Server environment. Note: Microsoft Terminal Server is supported for QuickBooks Enterprise Solutions only. Intuit does not support any other version of QuickBooks in a Terminal Server environment. Additional information on QuickBooks support for Microsoft Terminal Server can be found here.