That entirely depends on your plans for the mead.
Do you want it to be very “orangey”? Are you willing to let it age for a while? How long will it sit on the orange peel?
For myself, I usually only want the orange to be on the subtle side. Otherwise it could add a lot of tannins and tang. I only use it in secondary so it can…
1. Sit for at least 2 weeks
2. Use the alcohol from primary to extract the flavor.
For the methods described above, I usually use the peel from 1 large orange for a 3-5 gallon batch. That being said, I’m not 1 to let things over age, you however might be willing to let something be for a year or more. In that case the flavors will soften up and settle out. Rind from 1 large orange could be real nice in a single gallon.