Greetings,
AFAIK, there is no tool from Azure to capture the remaining IP addresses in a Vnet/Subnet that are ready for consumption. This feature has to be added In our networking API to GET those results.
You can provide your feedback here in this feedback section or upvote for the same. All the feedback you share in these forums will be monitored and reviewed by the Microsoft engineering teams responsible for building Azure
----------
Please do not forget to "Accept the answer" wherever the information provided helps you to help others in the community.