How many plants you will need is going to be dependent on a lot of factors: what type of tomatoes you're growing, the quality of your soil, how well you take care of your garden, your climate, the type of sauces you want to make, how much of those sauces your family eats, and so on and so forth.
In general, 4 tomato plants will yield 7 quarts of a basic spaghetti sauce. (Figured at 10 lbs per plant yield, and 40 lbs tomatoes needed per 7 quarts of sauce.) Some tomato plants will yield more heavily than that, some will yield lighter, some tomatoes contain more water and you have to use more, etc, etc, but it's a good place to start.
Personally, for our family of four, 20 plants is not even close to enough. We eat a lot of tomatoes though. Between canned tomatoes, salsa, spaghetti sauce, ketchup, bbq, tomato sauce and tomato paste it adds up quick.