I know that you can only win 1 card per merchant but now that i hit the skelly isles they have stopped after the first few towns. Not sure if this is because i have won all the cards i can other then specific ones from quests and or have to buy from inn keepers and such. Just curious if anyone else has run into this.

I have the same issue after playing a few games in Skellige. Suddenly I won some random loot and then I stopped winning anything apart from the bet. How do we know if we have all cards?

If you have stopped winning cards from merchants it means you have exhausted that way of gaining cards. The only other way is through quest related characters or buying them of certain vendors.