How many APU does an A320 have?

Excuse me, could you clarify something for me? I'm curious about the specifications of the A320 aircraft, specifically regarding its Auxiliary Power Unit, or APU. I'm wondering, how many APU does an A320 typically have installed? Is it a single APU system, or does it have a backup APU for redundancy? I'm trying to get a better understanding of its power supply architecture for emergency situations and regular operations. Your insight would be greatly appreciated.