How many dried herbs are mixed with a kilo of tomato sauce?
There is no set amount or rule to guide you, as there is a lot of variation in tomato sauce for spaghetti.  Some people use oregano, some use marjoram, some use thyme, some use basil...some use a combination.  Oftentimes fresh herbs are used, rather than dried herbs.  A tablespoon of dried oregano sounds like a lot to me, unless you really like that flavor.  I would start with much less, see how you like the flavor, then add more until it is of your liking.  It is much more difficult to remove or alter the flavor if you start with too much.  I would suggest you begin with a quarter teaspoon and work your way up.