The amount of rainwater that can be collected from your roof depends on the size of the roof and the amount of rain you get.
But if your roof is around 1000 square feet you can collect on average of 550 gallons of rainwater per each 1000 square feet and inch of rain.
So if you have a 1,000 square foot roof you can easily collect 550 gallons of free water if you get i inch of rain and if you have 2 inches of rain you can collect around 1,100 gallons of free rainwater that can be used for gardening or other things.
I collect rainwater myself and I get 550 to 600 gallons of rainwater from my 1,000 square foot roof depending on how much rain we get.