I've found that 3 eggs per 1 cup of milk is the ideal ratio of eggs to milk when making French toast. This would yield enough batter to soak about 8 slices of bread. Batter made with just 2 eggs per cup of milk will be very liquidy and make the bread a bit mushy. Conversely, batter made with 4 eggs per cup of milk is overly eggy and creates a dense French toast. Three eggs per cup of milk is the perfect middle ground.