Couatls can change shape willingly as an action
The stats for Couatls are available in the basic rules and are free to access on D&D Beyond (p. 269 in the SRD).
Listed under 'Actions', the Couatl has the following ability:
Change Shape. The couatl magically polymorphs into a humanoid or beast that has a challenge rating equal to or less than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the couatl's choice).
In a new form, the couatl retains its game statistics and ability to speak, but its AC, movement modes, Strength, Dexterity, and other actions are replaced by those of the new form, and it gains any statistics and capabilities (except class features, legendary actions, and lair actions) that the new form has but that it lacks. If the new form has a bite attack, the couatl can use its bite in that form.
This ability doesn't have a specified number of uses, and is not a spell. Therefore it is an at-will 'Action' that the Couatl can perform as often as it wishes (within the usual constraints of one action per combat round).