I want to develop an application that allows lots of users (hopefully in the area of 100,000) to call my system. I cannot put them all in iax.conf, so I need a method of validating them through a database. Is this possible?
yes it is. You want ARA, the Asterisk Realtime Architecture. It lets you store IAX/SIP/Extensions data in a database, which is then queried as data is needed. Thus, you can have any other system (ie PHP management portal or website) work on the database while Asterisk is running and changes take effect w/out reloading.
You might also be interested in: voip-info.org/wiki-Asterisk+Dimensioning . Keep in mind many of the stories here are not hard limits, but what was tested. IE, I have 5 calls running on my P-200! doesnt mean the thing dies if you send 6, but that he only bothered to try 5.