There are 3 main ways to manage Google My Business listings.
The first method is taking a normal Gmail account and using it as your “master” account. This is considered the riskiest method. Google can and will not hesitate to suspend a listing or any listings in the same account as a listing in question. Google can even take it a step further and completely suspend the Gmail account itself. This will not only suspend all your listings, but it will suspend your access to any google service such as youtube or your mail. Like all suspensions, you can request Google to review, but Google doesn’t necessarily have to restore your account until you fix what’s wrong, and Google will never tell you what you did wrong.
The second method is very similar to the first method. You create an agency Gmail account to manage your listings. While one might say an agency account is safe, it's still one master account. A listing breaking the rules is going to suffer the same results whether it’s in an agency account or a normal account.
The 3rd method is the absolute safest method. You want to separate the entities you manage into separate Gmail accounts. An entity that has an office, practitioner, and department listings is still one entity and should be stored on the same Gmail account. Once you have all your entities stored in separate Gmail accounts, you might feel that logging in and out of hundreds of gmails is quite annoying...and it is! But you can use a program such as Local Viking to manage all your Gmail accounts and all your listings in one master dashboard. This is the safest and most scalable solution to managing multiple GMBs for multiple entities.