How to balance an encounter with Challenge Rating zero creatures against PCs?

Question

The Homunculus is described as a Challenge Rating (CR) 0 creature with 5 hit points and a poison attack that, if PCs fail a DC 10 Con save by 5 or more, causes unconsciousness for 1d10 minutes.

According to the "Challenge" section in the intro of the MM (p. 9):

A monster's challenge rating tells you how great a threat the monster is. An appropriately equipped and well-rested party of four adventurers should be able to defeat a monster that has a challenge rating equal to its level without suffering any deaths. For example, a party of four 3rd-level characters should find a monster with a challenge rating of 3 to be a worthy challenge, but not a deadly one.

As we understand it, a CR1 creatures is considered an even match for four level 1 PCs. A CR 1/8 creature would need eight creatures to be an even match for level 1 PCs.

That math doesn’t seem to work out for CR0 creatures as it seems to imply an infinite number of creatures would be required to create an even match.

How does one calculate an even match with CR0 creatures? Given the Homunculus’s rather powerful poison attack, how many Homunculi would be an even match for a party of Level 1 PCs?

Answer 1

Use the encounter building guidelines in the Dungeon Master's Guide

Encounter building tools can be found in the Dungeon Master's Guide, page 81 on. They can also be found in DnD Beyond (which can also do the math for you).

For example, an encounter against ten homunculi would have an adjusted XP value of (2.5 × 10 × 10XP) = 250XP, where 2.5 is the multiplier for 7-10 hostiles, 10 is the number of the homunculi, and 10XP is the experience value of a single homunculus. For four level 1 PCs, this encounter would qualify as Medium.

To get the XP value for monsters in an encounter, you can use the CR to XP table on Monster Manual page 9. Each monster's XP value is also included in its stat block. CR 0 monsters are worth 0 or 10 XP depending on whether they have a proper attack or not.

The slightly modified encounter building scheme of Xanathar's Guide to Everything (also available in DnD Beyond) presents a simpler, not quite equivalent form:

For low challenge ratings not appearing on the table, assume a 1:12 ratio, indicating that twelve creatures of those challenge ratings are equivalent to one character of a specific level.

Remember that while the encounter difficulty calculation is a useful tool in normal situations, it's always the best to weigh the output against the strengths and weaknesses of the particular party you are placing the encounter against.