The maximum I would consider would be (5) SPA525G2 phones and all could not be in use at the same time. I think you could probably use (4) simultaneously without issue. The constraining factor is that you are using 802.11g on the phone to connect to the voice VLAN. This limits you to only a 54mbps data rate from the phone to the accesspoint in a best case scenario.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...