How do you double jump in a 2d game

this is the script I used for the character to move and jump etc. but I cant figure out how to double jump it is really frustrating.

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(PlayerPhysics))]
 public class PlayerController : MonoBehaviour {
     
     // Player Handling
     public float gravity = 20;
     public float walkSpeed = 8;
     public float runSpeed = 12;
     public float acceleration = 30;
     public float jumpHeight = 12;
     
     private float animationSpeed;
     private float currentSpeed;
     private float targetSpeed;
     private Vector2 amountToMove;
     
     private PlayerPhysics playerPhysics;
     private Animator animator;
     
     
     void Start () {
         playerPhysics = GetComponent<PlayerPhysics>();
         animator = GetComponent<Animator>();
     }
     
     void Update () {
         // Reset acceleration upon collision
         if (playerPhysics.movementStopped) {
             targetSpeed = 0;
             currentSpeed = 0;
         }
         
         // If player is touching the ground
         if (playerPhysics.grounded) {
             amountToMove.y = 0;
             
             // Jump
             if (Input.GetButtonDown("Jump")) {
                 amountToMove.y = jumpHeight;    
             }
         }
         
         // Set animator parameters
         animationSpeed = IncrementTowards(animationSpeed,Mathf.Abs(targetSpeed),acceleration);
         animator.SetFloat("Speed",animationSpeed);
         
         // Input
         float speed = (Input.GetButton("Run"))?runSpeed:walkSpeed;
         targetSpeed = Input.GetAxisRaw("Horizontal") * speed;
         currentSpeed = IncrementTowards(currentSpeed, targetSpeed,acceleration);
         
         // Set amount to move
         amountToMove.x = currentSpeed;
         amountToMove.y -= gravity * Time.deltaTime;
         playerPhysics.Move(amountToMove * Time.deltaTime);
         
         // Face Direction
         float moveDir = Input.GetAxisRaw("Horizontal");
         if (moveDir !=0) {
             transform.eulerAngles = (moveDir>0)?Vector3.up * 180 :Vector3.zero;
         }
         
     }
     
     // Increase n towards target by speed
     private float IncrementTowards(float n, float target, float a) {
         if (n == target) {
             return n;    
         }
         else {
             float dir = Mathf.Sign(target - n); // must n be increased or decreased to get closer to target
             n += a * Time.deltaTime * dir;
             return (dir == Mathf.Sign(target-n))? n: target; // if n has now passed target then return target, otherwise return n
         }
     }
 }