How to hook up multiple horn pa speakers.

Trying to upgrade a legacy pa system. A voip is also being used and there is a Cyberdata paging adapter to convert digital to analog. and am thinking about buying one of the Bogen TPU series amps to drive 12 horn speakers. The big ? is do I use some type of attenuator to "impedence" match or some other device to be able to hook all 12 to the amp? Or can I hook them up in a series/parallel combination to match the impedeance of the amp?
Mike