As in everything in life, there are limits. Below is a list of the various limits throughout the mission editor and the game.
Each mission is created on a grid that can be a maximum of 62x62 units square. The size can vary and is shown by a solid green line around the map...
You can have a maximum of 32x Areas and only 16x Switches.
You can have a maximum of 128 Triggers per side.
The condition Every NN Seconds accepts any interval — it is not limited to 99 seconds (older editor versions capped the input field at two digits, a UI limit only).
There are some recommended design limits for missions.
Computer Structure Limit : 52
Computer Mobile Unit Limit : 80
Human Structure Limit : 39 + unused Computer amount - 5
Human Mobile Unit Limit : 60
Example : If the computer only has 36 structures (buildings, towers, etc), then the human player can have 39 + (52 - 36) - 5 = 50.
There is also a limit on the number of scenery items that can be used (This only includes bushes, trees, etc. Not roads and rivers). This limit is 100 items