I like to grate vine-ripened tomatoes with a handheld grater right into my Pyrex quart measure (could use a mixing bowl). I grate until I’m left with a piece of skin in my hand, and then discard the skin. Optionally, you could use 4 cups of canned crushed tomatoes or even a mix of canned and fresh. When using fresh, the number of tomatoes needed will vary based on size. My last batch used six. Start with more than you need, weight-wise, as you will be discarding much of the skin. Optionally, you could use Roma or plum tomatoes, which tend to be less watery. This is fine. The desired thickness of the ratatouille can easily be adjusted in the cooking stage.
To freeze ratatouille, cool it completely and then transfer to an airtight container and freeze for 3 to 4 months.