Depends entirely on the goose. I have had geese go broody after laying only 5 eggs, and then others who lay over a dozen. However, I would not encourage broodiness just yet. Goslings hatched in mid-late May get the benefits of the warmer weather and better grass.
My method is to remove all eggs laid until at least the beginning of April, then start leaving the eggs laid in one particular nest and DO NOT MOVE. Once that nest is being covered by the same goose pretty much all day long erect a barrier between her and the rest of the flock so no more fresh eggs are being laid into that nest and she gets peace and quiet to brood.
As you know goslings can take up to 32 days to hatch so it's a bit of a wait but I just leave them to it. They are not as easy as broody hens and best left alone as they can give you a real bashing if they want to and can break the eggs just trying to defend the nest. People are often nervous about ganders but a broody goose is far more ferocious! Give them a bucket of water nearby and offer food twice daily (remove if not eaten) and that will be all they need. If you have the space give them their own personal bath. They won't use it very often but maybe once a week they will appreciate a splash about.
When goslings hatch after about 24 hours place a bowl of waterfowl grower pellet close by so they can discover it (you will have to approach the nest very carefully not to agitate mum). The goose will teach them everything else they need to know.