the proper ammount is probably 3-4 for a player with decent active time. (4 being more luckey) Im currently 12x HC NM, 12 Clears Normal NM, 6 clears ToV NM and 3 clears toV HC, 4 Normal Nighthold, and some heroic kills and im still sitting on 2 legendarys.
Mind you that i also run mythic + for weekly chest and does my emessary