To found or conquer a new village you need a free expansion slot. To check whether your village still has an open slot you just need to go to your residence or palace and open the "Expansion" tab therein.

Every village has an exact defined number of available expansion slots: 2 in case of a residence and 3 in case of a palace built. These slots belong to the village itself.

What does this mean for you?

In case you conqer a village, it can be that the old owner already founded one, two or three village(s) from it. Player 4seasons conquered a village of player bitter-sweet in the examples shown below:

As you can see ,there are already 2 expansion slots used by player bitter-sweet.

If this happens to you, then you will only be able to found as many villages as you have open edslots. So if he found two villages already (as in the example shown above) then you cannot found or conquer anymore villages with a residence, but you can found or conquer one more village with a palace level 20.