NewMonoBehaviourScript.cs

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

public class NewMonoBehaviourScript : MonoBehaviour {
    public List<Button> buttons = new List<Button>();

    void Awake() {
        foreach (var button in buttons) {
            button.onClick.AddListener( OnButtonClick );
        }
    }
    void OnButtonClick() {
        //log the position in the list of the button that was clicked
        int index = buttons.IndexOf
        (
            (Button)UnityEngine.EventSystems
                .EventSystem.current.currentSelectedGameObject
                .GetComponent<Button>()
        );

        Debug.Log( "Button clicked: " + index );
    }
}