There is not a "number". What is needed depends on your circumstances and what you actually have. For those who have not lived together yet, concentrate on direct evidence of being together in person. Boarding passes (not itineraries) and passport stamps. Hotel receipts are good too. Photos together are good but secondary. If you multiple select photos from a folder, select nine then print to wallet size on plain paper nine per page. Make it a variety. Three pages of photos like that seems ideal. You cannot provide something that doesn't exist. Don't try to contrive anything. It is what it is.