Hello Wang, Zhipeng
Currently, Azure Key Vault does not provide a direct metric for measuring the total storage consumed by secrets, keys, or certificates(I am not sure if it will in the future)
Key Vault does not restrict the number of keys, secrets or certificates that can be stored in a vault. The transaction limits on the vault should be taken into account to ensure that operations are not throttled.
Key Vault does not restrict the number of versions on a secret, key or certificate, but storing a large number of versions (500+) can impact the performance of backup operations.
https://learn.microsoft.com/en-us/azure/key-vault/general/service-limits