What is the largest number of Settlers expansions that can be combined in one game?

Question

I recently obtained the Traders and Barbarians expansion for Settlers and in the rules for each sub-expansion there was a little addendum about possible combinations with other sub expansions. But really, how many expansions could you theoretically combine without creating rule conflicts?

An example of a rule conflict would be if two expansions each use their own development cards, or if they each require specific incompatible board shapes.

Of course, if a third rule exists that describes how to handle these rule conflicts, then there is no conflict.

Answer 1

There are 4 official expansions; which can legally all be used at the same time.

3 of them, Seaside, Cities & Knights, and Traders & Barbarians can all be combined with any number of them being used or not used.

The 4th, Explorers and Pirates, does not combine as well, because it changes things more than the others in ways that conflict with the other expansions. The rules allow you to combine it with others, but the game's publisher recommends only using it as an independent expansion.

Cities & Knights comes with its own mechanic to replace development cards, so you simply do not use the development cards from the base game. The official FAQ has clarifications on how to handle things that only come up when combining expansions.

Note that both Explorers & Pirates and Traders & Barbarians are modular expansions, each with several different scenarios that use different mechanics. Some scenarios combine better with the other expansions than others. Catan.com lists each of the scenarios and how they work with the other expansions:

Traders & Barbarians combinations

Explorers & Pirates combinations