Do not assume that your local authority will provide you with sandbags in an emergency! Sandbags will be in short supply and local authorities will choose to protect vital infrastructure such as hospitals, substations etc before residential properties. If you live in an area prone to flooding it is a good idea to be well prepared before any flooding. Sandbags are of no use to protect a property if it has already been flooded.
When working out how many sandbags you need do not forget all the entry points into your house. Airbricks, cable entry points, utility meters, cat flaps etc will all let water into your property if they are not protected.
You will need at least six sandbags to protect a standard door from 20cm depth of flood water.
Placing your sandbags
For more information please see Building Substantial Walls