The number of vSphere clusters and the total of virtual machines are important information related to the virtual infrastructure that are missing and must be collected during the discovery phase. These information can help you estimate the backup performance, scalability, and resource requirements for the Veeam backup infrastructure. For example, you can use the number of vSphere clusters to determine how many Veeam backup servers and proxies you need to deploy and how to distribute the backup load among them. You can also use the total of virtual machines to calculate the total amount of data to be backed up, the storage space required, and the network bandwidth needed.
References: [Veeam Backup & Replication Best Practices], [Veeam Backup Infrastructure Sizing Calculator]