There are advantages and disadvantages for servicing many brands.
advantages... most of all you have a wider spectrum of potential customers and that means more business and therefore more income which equals more profit and if you share your profit, your employees will be more happy and a win-win situation.
disavantages... techs have to carry more parts, more time to research, more dealer authorization, more human memory. your parts warehouse database becomes huge and parts machines stock bigger. more disgruntled techs, especially slower ones. of course there are many more factors, but this is what i can think of now.
I would say four to five brands is sufficient with two out of those being more dominant in numbers.