One or Many Vnets?

Hi, Difficult to give a full response with the limited information, however I would suggest looking through the zero trust model, in particular the assumption of compromise and the assumption of breach. With the addition of multiple vNets, it will reduce the blast radius should it get breached and make it easier for our friends in the compromise recovery team to help the customer.

I hope this article will help you; https://docs.microsoft.com/en-us/security/zero-trust/networks