Yield: 4 x half-litre jars (US pint)
You will end up with tomato sauce equalling about 2 litres / 8 cups / 64 oz. If you have lots more than that, your variety of tomatoes may have had a lot of water in them -- keep on simmering the sauce to get it down to roughly around this amount. (You may wish to do up till here the day before, then refrigerate overnight, and bring back up to a boil the next day and proceed.)
You can use a food processor to finely chop the onion.
Yes, you must peel the tomatoes to reduce the bacterial count for safe canning: most of the bacteria is on the skin.
How long the simmering to thicken it will take varies wildly. The broader the pot, the faster it will reduce.
Tomato Ketchup. www.healthycanning.com