secondaryPrivateIpAddressCount

The number of secondary private IPv4 addresses to assign to a network interface. When you specify a number of secondary IPv4 addresses, Amazon EC2 selects these IP addresses within the subnet's IPv4 CIDR range. You can't specify this option and specify more than one private IP address using privateIpAddresses.

You can't specify a count of private IPv4 addresses if you've specified one of the following: specific private IPv4 addresses, specific IPv4 prefixes, or a count of IPv4 prefixes.