The document you pointed says -
Our recommendation is to use a vault per application per environment (Development, Pre-Production and Production). This helps you not share secrets across environments and also reduces the threat in case of a breach.
So ideally you can create 3 Key vaults - Dev, UAT and Prod. This is what I have seen in real projects.
All the (20 or 30) applications in Dev environment can fetch secrets from Dev Key vault and developers can be granted access to this Key vault. Creating 20-30 Key vaults would be complicated and difficult to manage.
For UAT & Prod keyvault, only admins should have access.
Please don't forget to Accept Answer and Up-vote if the response helped -- Vaibhav