There is no answer to how much do you call. Or to put it another way. However much the geese want. You need to learn to read the birds and then act like a politician and tell them what they want to hear.
Start out with very little just to get their attention and over time as you read the birds better and get more confident in your calling you will have a better feel of what to do.
Single geese, I've had pretty good luck just echoing back what they are saying to me.
Flagging is also very effective, but it will focus the geese right on you so unless you are well hid in the middle of the decoys, don't do it except to get their attention when they are far away. They can see that flag a longggggggggggggggggg ways off.
If you shoot a goose, plead for their mate to come back. Hoooooooooooonk, hooooooooonk, ... You will be surprised how often you will turn them around and bring them back after you kill a goose.