What are the signs of an understaffed DevOps team?
There are four main reasons why you can feel your team is understaffed:


Poor organization and planning of work
Doing work someone else should be doing
Doing work that should not be done at all
Being actually understaffed


Start with a review of the first three points. Read the Phoenix Project on ideas how to do the first. Ask yourself for every task you help anyone with if it should be done at all and if it is you that should be doing the task or if you should simply enable whoever needs it done to do it themselves. This will give you some documentation on why all the work you do is necessary.

Next review the four types of work mentioned in the Phoenix project:


Business Projects - what you do for other teams in the organization
Internal Projects - what you do to make your work easier in future
Scheduled Maintenance - what you do to keep the lights on
Unplanned Interrupts - what you do because something went wrong


If the work of your team is sustainable, you will spend roughly the same amount of time on each of the four. If the unplanned work starts to creep up close to 50% of your time, it is a sign you are definitely understaffed. 

You should be able to hire to stay about one person ahead of the unplanned work reaching 25% of your time, otherwise, one person leaving will send your entire team into a tailspin you might never recover from. Overprovisioning of people and technology have same reasons and benefits.