While the subnet doesn't have to be reserved for the SDDC, it is best to do dedicate one for it. There are certain constraint on the route tables and security groups being used.
The subnet for the SDDC shouldn't be to big, and a /26 is recommended, but a /27 can be used as well.
More can be read here:
https://blogs.vmware.com/cloud/2019/10/03/selecting-ip-subnets-sddc/
