AnimatorOverrideController.overridesCount

Возвращает число переопределений.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ResetOverrides : MonoBehaviour
{
    public AnimatorOverrideController overrideController;
    protected List<KeyValuePair<AnimationClip, AnimationClip>> overrides;

    public void ResetAllOverrides()
    {
        overrides = new List<KeyValuePair<AnimationClip, AnimationClip>>(overrideController.overridesCount);
        overrideController.GetOverrides(overrides);
        for (int i = 0; i < overrides.Count; ++i)
            overrides[i] = new KeyValuePair<AnimationClip, AnimationClip>(overrides[i].Key, null);
        overrideController.ApplyOverrides(overrides);
    }
}