Typically ALPACA is administered in groups of 4-6 using best assessment practice in terms of set up, planning and a sensible quiet environment where the screener is carried out, with minimal distractions- same as you would any other assessment/screener.
There are rich observational insights to be gleaned when as a practitioner you don't have to administer the screener yourself. We have had schools that have taken much larger groups and put more than one staff member in the room to supervise the infants and assist in getting them set up.
We'd also suggest to have a game on the devices for the early finishers, which would become all the more important if you decided to screen in larger groups.