If you were going really simple and wanted a 1 kite quiver, it would probably be a 9m or 10m. It's the in-between size. Looking at my sailing log stats, it's the most used kite (varies by location of course).
But then if it's light, you get to watch your buddies having fun if you don't have a 12m. Worse, when it's cranking and really good, you have the really difficult question: has it crossed the line into dangerous on my too-big kite? So a multiple kite quiver is ideal, if you can afford it.
Something to take the sting off: if you buy the 3rd kite, it reduces wear and tear on each of the other two kites by 50%, so they can last a lot longer between replacement. Economics 101 says buying more kites actually SAVES you money (there's the argument for your spouse, hehe)
I have a 2m gap between sizes, which makes for a lot of kites. I think a 3m gap is ideal.....Ocean Rodeo has had the standard sizes 7m, 9.5m, 12m, which seems a pretty good coverage of the normal wind range. Then you can get into 5m and 17m for the abnormal wind ranges too.
One final consideration: using foils, you are usually on a kite size smaller. A lot of foilers don't use 12m kites any more, so that can potentially pay for a foil.