Practically all UK businesses need extinguishers. Technically, a fire-risk assessment should decide if they are necessary but UK government advice tells us that even a one-room shop should have at least one multi-class extinguisher.
However once extinguishers are installed in a business it is then a requirement to have them serviced every year.
How many extinguishers do you need? There are specific requirements but there are some simple guidelines: The extinguishers firstly should be appropriate to the potential fuels (e.g. having just a water extinguisher is no good if you have electrical equipment). The extinguishers should be positioned within the required travel distances (i.e. you should not travel more than 30m to find an extinguisher).
BS5306 states that they should be positioned in prominent positions and recurring positions on each floor and that each floor should have extinguishers (i.e. you should not have to travel between floors to locate an extinguisher).