Amount of yeast is less important as the type of yeast and ingredients in the must to make a sweet mead. If you want to limit the amount of yeast used up front then 1 gram of well hydrated yeast can start a ferment but if I purchas a standard 5g pack of wine yeast and know I will use it soon on another must then I'll pitch half a pack. Otherwise I just pitch in the full 5g pack.
What yeast are you wanting to use and what kind of recipe are you following? The more details you give us the better we can help.