A large riding establishment/livery yard with horses used for most disciplines including cross country and show jumping would have a greater need for first aid provision than a small livery yard where staff had minimal direct contact with the horses.
Many yards will have small numbers of employees but it is still deemed necessary to have at least one first aider available at all times. For establishments covering provision to the public it would be reasonable to expect to have a first aider present at all such times.
Medi-K training offers this HSE complaint 1 day Emergency First Aid at Work Course which will give a complete overview of first aid procedures. It is specifically designed for those who are involved in horse riding activities, including events, leisure and recreation.