Welcome to playing against husk simulator. Instead of waiting for a merchant to put random 5 cost cards in your hand, we're putting the cards in our hand on purpose.
Lands are complicated, but usually with mistral you want deserts by your face and at least one mountain to set up axegrinder dash. Merchant may motivate you to go beyond 4 lands, but 2 of each is all you need to start.
*an occasional map might slip through the cracks