DMG Table 54

Table 54: 2-20 Encounter Table
DiceRoll	Frequency
2 	Very rare
3 	Very rare
4 	Very rare or rare (DM's choice)
5 	Rare
6 	Rare
7 	Uncommon*
8 	Uncommon*
9 	Common**
10 	Common**
11 	Common**
12 	Common**
13 	Common**
14 	Uncommon*
15 	Uncommon*
16 	Rare
17 	Rare
18 	Very rare or rare (DM's choice)
19 	Very rare
20 	Very rare
20 	Very rare
:* Or choice of two very rare creatures, 50% chance of each.
:** Or choice of two rare creatures, 50% chance of each.