This is an impossible calculation; rainfall varies, and any statistics available are merely average figures. Equally, calculating the amount of water your garden and its plants are likely to need in any given year is just as impossible - it depends on the weather, the size and type of plants you have, and other factors, such as how recently they've been planted, whether they fruit and so on. Usually, people just set up a rain collection system where they can to collect what they can, by attaching a diverter to a downpipe from the guttering on the roof.
You are also (or should be) restricted in regard to water collection by possible health risks; your water butt or barrel is best restricted to no more than 100 gallons capacity, and out of sunlight to help stop the water becoming hot during hot weather. The risks include legionella, campylobacter, salmonella (usually from fecal contamination off the roof) and some other possible infection and contaminant risks. Storing too much water in one container, which if its very large will be difficult to clean regularly, is not a good idea. There may or may not be guidance and/or restrictions in your own country which apply to collection of rainwater, so it might be helpful to check with whatever Environmental body you have there to see what their recommendations are.
An overview and some information in this PDF document which might be helpful