A good rule of thumb is to add 1 primary category and 2 secondary categories max to any one GMB listing. You can add more categories but each category needs to have content backing it up. Otherwise, you risk losing your Knowledge Graph in the SERPs.
An example is, if you're a general practice law firm, your primary category would likely be Law Firm and your secondary categories would be dependent on what your site speaks about. If your site talks about Family Law and Divorce cases, you would add Family Lawyer and Divorce Lawyer as secondary categories. Maybe your site speaks about Personal Injury as well. Go ahead and add Personal Injury Attorney as a secondary category. But if your website doesn’t speak about Immigration Law, do not add Immigration Attorney as a category.