So just how many First Aiders do you need?

I suggest you that you steer clear of those online “how many first aiders do you need” calculators found on many Training Company websites. Simply because the Health and Safety Executive guidance is that you should look at your organisation and ask some sensible questions:
• How many employees/customers do you have?
• How many locations do you operate in?
• How long would it take to get the emergency services there?
• What shifts do employees work?
• How dangerous is their work environment?
Once you know the areas that you need to look at and how likely there is to be a First Aid incident, you then have to decide if you have enough cover. This is called a “First Aid Needs Assessment” and it is your responsibility, you will have to justify your decisions if something goes wrong and you didn’t provide sufficient first aid.
Rather helpfully the Health and Safety Executive does provide some case studies to guide you. For example, if you have an office with eighty employees spread evenly over two floors working normal business hours with good access to emergency services, you would always need at least one first aider on duty on each floor, fully trained on the one-day first aid course (Emergency First Aid at Work). To cover for absence and holidays you would have to train at least four employees or possibly more.
Another case study given is a construction maintenance crew of twenty working in teams of two, sometimes working at height, with electricity etc., travelling in transit vans up to ten miles from the nearest hospital. In this case you would need at least one first aider per team trained, up to the three-day first aid course (First Aid at Work). Once again, to cover for staff holidays and absence you would probably have to have at least twelve team members trained.
The area of the Health and Safety Executive website dealing with First Aid is worth a visit as it has a lot more case studies in greater detail as well as lots of clear guidance written in plain English.