There is one: Deathlock Mastermind (MToF, pg. 129).
The Deathlock Mastermind's attack Grave Bolts says:
Grave Bolts. Ranged Spell Attack: +6 to hit, range 120 ft., one or two targets. Hit: 18 (4d8) necrotic damage. If the target is Large or smaller, it must succeed on a DC 16 Strength saving throw or become restrained as shadowy tendrils wrap around it for 1 minute. A restrained target can use its action to repeat the saving throw, ending the effect on itself on a success.
I have searched through all three primary monster source books and this is the only one I found.
Honorable mention: There is another example of a creature that specifies something different than simply "one target" for actions requiring attack rolls. The Mind Flayer (MM, pg. 222) and its variations1 have the Extract Brain action, which specifies,
one incapacitated humanoid grappled by the mind flayer.
This is still a single target, but it does represent a departure from the near universal "one target" for actions that use attack rolls.
1Mind Flayer Psion (VGtM, pg. 71), Mind Flayer Arcanist (MM, pg. 222), and Mind Flayer Lich (VGtM, pg. 172).