Check out example codes for "unity how to make jump script". It will help you in understanding the concepts better.

Code Example 1

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

public class jump : MonoBehaviour
{
    public float jumpHeight = 7f;
    public bool isGrounded;
    public float NumberJumps = 0f;
    public float MaxJumps = 2;
    private Rigidbody rb;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }

    void Update()
    {
        if (NumberJumps > MaxJumps - 1)
        {
            isGrounded = false;
        }

        if (isGrounded)
        {
            if (Input.GetButtonDown("Jump"))
            {
                rb.AddForce(Vector3.up * jumpHeight);
                NumberJumps += 1;
            }
        }
    }

    void OnCollisionEnter(Collision other)
    {
        isGrounded = true;
        NumberJumps = 0;
    }
    void OnCollisionExit(Collision other)
        {
            
    }
}

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