The encounter rate is balanced to be realistic (according to the source material), I can get on floor 5 by encountering around 10 mobs (Which is used as a metric for many of the main quests). It's not affected by difficulty (Although Normal locks the maximum monsters in a fight to 5).
If you want to increase or stop encounters, you need to buy the necessary items at the market: "Monster Lure" to increase the encounter rate to a near 100% and "Morbul" to set the encounter rate to 0%.