HELP!! I don't know how to do the 2Dcharacter jump only once in C#code

I don't know how to set jump , anyone can help me?please!!

 using UnityEngine;
 using System.Collections;
 
 public class Charectercontrol : MonoBehaviour {
     public float maxSpeed = 5f;
     bool facingRight = true;
     Animator Anim_wait;
     public float maxjump = 20f;
     Rigidbody2D rigidbody2d;
     public bool grounded;
 
 
     // Use this for initialization
     void Start () {
         rigidbody2d = GetComponent<Rigidbody2D>();
         Anim_wait = GetComponent<Animator>();
     }
     void FixedUpdate ()
     {
         float move = Input.GetAxisRaw ("Horizontal");
         bool jump = Input.GetKeyDown (KeyCode.UpArrow);
         if (move != 0) {
             Anim_wait.SetBool ("run", true);
         } else {
             Anim_wait.SetBool ("run", false);
         }
 
         rigidbody2d.velocity = new Vector2 (move * maxSpeed, rigidbody2d.velocity.y);
         if (jump) {
 //            case Anim_wait.SetBool("jump", true):
             rigidbody2d.velocity = new Vector2 (rigidbody2d.velocity.x, maxjump);
 
         }
         if (move > 0 &&! facingRight)
             Flip ();
         else if (move < 0 && facingRight)
             Flip ();
     }
     void Update(){
 
     }
     void OnGUI(){
         GUI.Label(new Rect(0,0,100,30),rigidbody2d.velocity.ToString());
     }
 
     // Update is called once per frame
     void Flip (){
         facingRight = !facingRight;
         Vector3 theScale = transform.localScale;
         theScale.x *= -1;
         transform.localScale = theScale;
     }
 }