Theoretically, there is no maximum. If you need registration/authentication, you can do this for up to 6 registrars beginning with IOS 15.0(1)XA. Prior to this release, you could only register/authenticate to one registrar. See the following link.
http://www.cisco.com/en/US/docs/ios/voice/sip/configuration/guide/sip_cg-multi-registrars.html
There are ITSPs that do not require registration/authentication. I have not seen a supported number of SIP trunks in this scenario as this is essentially just a voip dial peer in the configuration.
Hope this helps.
Brandon
