Why Must the Subnet Where the Load Balancer Resides Have at Least 16 Available IP Addresses for Enabling IP as a Backend?

These IP addresses are used by the ELB system. Generally, two IP addresses are required for creating a dedicated load balancer in a single AZ, and six IP addresses are required for creating a dedicated load balancer with IP as a backend enabled. If you create a dedicated load balancer in multiple AZs, more IP addresses will be required. There is an algorithm to calculate how many IP addresses are required.