How many campaign Starship cards can be in play, and how do their X and plant pairs work?

Question

Accepted Answer

At most three campaign Starship cards in total, chosen freely from those unlocked, not three sets. When a card pairs a Planning X with a plant action, write the X (it counts as a numbered space, not a Planning action) and then perform the plant action.

Welcome to the Moon

How many campaign Starship cards can be in play, and how do their X and plant pairs work?