Depending on the aircraft model and the number of passenger seats in the airline's cabin layout, there will be a regulatory minimum number of cabin crew required (based on the 1 per part of 50 passengers rule and the number of crew that were used in the aircraft manufacturers evacuation tests) and defining the required cabin crew positions (including assistance at emergency exits and for viewing throughout cabin).
An aircraft may indeed be fitted with additional cabin crew seats, which can then be occupied by additional cabin crew as necessary or by other authorised persons according to the airline's operational procedures.