Yes Limos can serve alcohol to the passengers.
However to be able to serve alcohol in a limo you need and alcohol serving license along with your license for the Limo.
The reason Limos can serve alcohol to passengers in the limo is there's a window in the limo that separates the drivers compartment from the passengers.
So the passengers can drink alcohol while the limo driver does the driving.
The people in the limo can get drunk as long as the limo driver does not drink any alcohol and any professional limo driver would not drink alcohol while driving.
I've drank alcohol in a limo before for a party and it was legal to do so.