DMG 274 suggests these values for AC as part of the table for defensive CRs (HP also factors into defensive CR, but is not listed here):
- CR 0-3: 13 AC
- CR 4: 14 AC
- CR 5-7: 15 AC
- CR 8-9: 16 AC
- CR 10-12: 17 AC
- CR 13-16: 18 AC
- CR 17+: 19 AC
According to the guidelines, the ultimate CR of a creature is the average of their offensive and defensive CRs. However, the DMG encourages additional tweaking and adjustments to individual monsters beyond the listed guidelines:
Alternatively, you can determine an appropriate AC based on the type of armor the monster wears, its natural armor, or some other Armor Class booster (such as the mage armor spell). Again, don't worry if the monster's AC isn't matching up with the expected challenge rating for the monster.
There are also a number of features that a monster can modify the effective AC for CR calculation purposes. These are tabulated starting on DMG 280. For example, giving a monster magic resistance boosts its effective AC by 2, which might result in a defensive CR that's higher than the one calculated strictly from HP and AC.
Thus, there is going to be significant variation in the AC of monsters at any CR, because there are so many factors that can change the final AC relative to the CR, including the judgment of the person designing the monster.