I dont have the rules handy but isn't there a rule for making individual checks that says if half or more succeed then the target is surprised? Maybe I dreamed that.
The DM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the DM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn’t notice a threat is surprised at the start of the encounter. If you’re surprised, you can’t move or take an action on your first turn of the combat, and you can’t take a reaction until that turn ends. A member of a group can be surprised even if the other members aren’t.
If I walk into my living room and notice that one of my brothers is hidden behind a couch, couldn't I still be surprised by the other brother hidden behind the other couch, who I didn't notice? Why would one brother automatically make the other brother known?
I would normally set the DC ahead of time for the encounter in a prepared ambush scenario. For an impromptu encounter I'd probably just roll once.[MENTION=6799649]
So how are DMs handling ambushes? Are most DMs rolling one Dexterity (Stealth) for a group of monsters trying to hide? Or are they rolling individually? I could see rolling individually for giant badgers hiding, but rolling once for a group of goblins who have orchestrated their ambush.
You only need to know the lowest Stealth, I roll once for a group and consider that the worst roll to save time, but you can always roll for each creature or subdivide the enemies in different groups to roll more dice but less than per individual.So how are DMs handling ambushes? Are most DMs rolling one Dexterity (Stealth) for a group of monsters trying to hide? Or are they rolling individually? I could see rolling individually for giant badgers hiding, but rolling once for a group of goblins who have orchestrated their ambush.
In the game I ran, a dagger of warning was the first magic item that the party found, and thereafter I didn't bother rolling Stealth checks.So how are DMs handling ambushes? Are most DMs rolling one Dexterity (Stealth) for a group of monsters trying to hide? Or are they rolling individually? I could see rolling individually for giant badgers hiding, but rolling once for a group of goblins who have orchestrated their ambush.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.