Gen Z Bees have a Mission Cap and a Mission Span. Both affect how many Missions the Bee can complete. (see the chart below for full details).

Mission Cap is the number of Missions the Bee can complete before he retires 

Each Gen Z Bee starts with a Mission Cap of 20.

The Mission Cap is increased each time the Bee is upgraded. Upgrading your Bee's stats will add 10 to the Mission Cap for the upgraded Bee.

Upgradable Gen Z Bee Stats are: Health, Attack, Defense, Agility, Luck, and Capacity.

NOTE: Any rarity upgrade will add 10 to the Mission Cap.

Each successful Hunt or Harvest adds one to the Mission Count

Once a Bee's Mission Count matches his Mission Cap, he will need to be upgraded to add to his Mission Cap.

If he doesn't have any open upgrade slots, he will be retired.

NOTE: Player vs. Player (PvP), Quests, and Breeding do NOT affect the Mission Count.

Mission Span is the number of days a Bee can be active on Missions

A Bee will retire once the Mission Span hits 0, even if he hasn't reached the Mission Cap 

NOTE: Mission Span cannot be extended

Starter Bees have a 90 day Mission Span

Hatched Bee have a 30 day Mission Span

Bred Bees' Mission Span is the Queen's Level x 3 (Minimum 30 days, Maximum 60 days)

NOTE: Retired Gen Z Bees can still be used to do Quests and PvP