30 phones in it's own broadcast domain (VLAN) should be fine, we have individual voice VLANs with hundreds of phones, as long as it is setup correctly there shouldn't be an issue...
How to set this up will vary based on the network configuration and networking equipment being installed, there are numerous ways to do this... via DHCP hand-off, Cisco SmartPort, LDAP, hard coding phones, etc, etc... Each instillation is unique, although there are general guidelines it will vary depending on any number of circumstances, and each tech here will probably tell you a different way to implement this, and they are all probably "correct".