The game works with the assumption that the party is made of 3-4 characters.
However, the game doesn't really need to be "balanced" for combat. Consider that the main driving force for Extractionists is not killing enemies, but rather exploration, mapping, and retrieving artifacts of the Spiral City. When players are facing foes that look too strong or too numerous, they should be encouraged to flee as fast as they can!
If you wish however to balance a certain encounter for plot reasons, you can add 1.3x enemies (rounding down) per additional player.
Example: you have a party of 4 players, about to face 9 Orcs. Another player joins in. 9x1.3=11.7, so they will now face 11 Orcs.
Hope that helps, if you have any other question, please ask!
