inSSIDer Office will give a Configuration Warning if too many virtual SSID's are present on the same access point radio.
Enterprise-grade access points have the ability to offer virtual SSID's, or more than one wireless network name per access point. Virtual SSID's are commonly used to provide access to different parts of the network, often for different departments.
For example, MetaGeek (awwww yeah that's us!) might offer WiFi for different departments (but to be honest, we don't):
Virtual SSID's requires the radio to send beacons and management overhead for each SSID on the radio, which can reduce airtime available for actual data.
Reduce the amount of virtual SSIDs to three or less.
Technical Background and Further Reading