Shoelace, your question can be likened to "how much water can a bath tub hold" it depends on the size or maybe how much will be needed at a given instant in time.
If you are asking what minimum size service is required, you need to read 230.79
I've never seen a 1200 amp resi service, but I have done a home (a 'McCastle', actually) with two 400 amp services (1 house, 1 garage) and a 200 for the shed and 200 for the driveway heat.
I'd hate to see the monthly bill on that place!