There is a rule in 5e which is applied when creating encounters: when you’re creating an encounter, you scale the total XP budget by a certain factor depending on the number of monsters. So if you have a single monster with an XP value of 100, then the encounter’s adjusted XP value is 100, but if you have a pair of monsters worth 50 XP each, then the adjusted XP for the encounter is 150. The point of the adjusted XP is to take into account the fact that multiple monsters making multiple attacks have multiple chances to do damage to a party; this protects parties from death of a thousand cuts (did you hear about the 5th level party that was decimated by kobolds?).
However, this only really works if you’re using enemies of a similar CR. I ran an encounter over the weekend with an Archmage (CR 12), a couple of Acolytes (reskinned Cult Fanatics – CR 2) and a horde of zombies (CR 1/4). The party was 7 level-8 PCs, and I was pitching the difficulty of the encounter somewhere between “hard” and “deadly”. My goal was to have a boss who would pose a major threat to the party (the Archmage), some support characters (the Acolytes), and some minions who would soak up the party’s attacks for a few rounds, and maybe do some damage. I was very fond of the “minion” class of enemies in 4e which could deal out level-appropriate damage, while not being punching bags, and I was trying to do something similar with these zombies. I specced up the encounter with http://kobold.club/fight/#/encounter-builder, and it told me that it would be a deadly encounter for my party.
The party destroyed the encounter. The mage was dead in three rounds of combat, and his acolytes didn’t even last that long (although they were able to get a few decent hits in). Startlingly, the zombies were the ones who lasted longest (thanks to undead fortitude), but unsurprisingly, they did the least damage. I guess the lesson that I learned here is that it’s better to use a smaller number of level-appropriate mooks than a large number of low-level mooks. It’s also worth thinking about XP scaling applies to PCs – 7 PCs focusing their attacks on the mage took less than two rounds to kill him.