How many missions are there in Zombie Farm? How are missions unlocked?

There are 12 missions in Zombie Farm Season 1, with each mission unlocked sequentially. Here is an illustration: there are three missions in level 1, then the user can pick any mission to start, taking 1-a as an example, then after the user completes the 1-a challenge, then the 2-a challenge is unlocked, and so on, continuing with the 3-a and 4-a missions.