You need to set the maximum number of people to go on leave because if not you will loose control of the performance. In doing this, you need to also consider the possible MC, EL and etc. Example: if you think at any point of time you can still cope with the volume if 4 people go on leave, you should set 2 as the max number for approval because there could possibly be 1 or 2 more will be on last minute MC, EL or etc. This is just an example, you would know best what is the right number.