Challenge Ratings
Pen, Paper & Pixel has a useful encounter calculator.
Fractional Challenge Ratings
You may be confused on looking through the Monster Manual and seeing creatures with listed CRs of 1/2 or lower.
Essentially, CR 1/2 is the equivalent of CR 0. For every halving, the CR decreases by 1, so that CR 1/4 is CR -1, and CR 1/8 is CR -2.
Adjusting Challenge Ratings for multiple creatures
If you have trouble calculating CRs for groups of equally powerful creatures, refer to the following table.
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Number of creatures | CR adjustment | 1 | +0 | 2 | +2 | 3 | +3 | 4 | +4 | 5 | +4 or +5 | 6 | +5 | 7 | +5 or +6 | 8 | +6 | 9 | +6 | 10 | +6 or +7 | 11 | +7 | 12 | +7 | 13 | +7 | 14 | +7 or +8 | 15 | +8 | 16 | +8 |
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You should never use more than 16 creatures in a single encounter - at that point the individual creatures are so weak that PCs would not receive XP for fighting them individually. If you want a fight against large numbers of monsters, use the "mob" template from the
Dungeon Master's Guide II, which is essentially a scaled-up version of a swarm.