Sherap, If you are using an IP address manager (IPAM), you should be able to run a report on used vs. available IPs. This will give you a very broad number, though. If you don’t have an IPAM solution, you can set up discovery sites with IP ranges to discovery your assets. You should also work with your network team to understand the ranges that are/are not used and to provide CIDR blocks for those that are and run a discovery only on those IPs.
DHCP logs will assist with correlating IP with hostname to avoid duplicates. So, if you can, use DHCP logs. This method may not help with static hosts and a support ticket could be used to get the details you need.