NumberSelect.cs

using UnityEngine;
using UnityEngine.UI;
using static UnityEngine.UI.InputField;

public class NumberSelect : MonoBehaviour
{
    [Header("References")]
    [SerializeField] InputField input;
    [SerializeField] Button increaseButton;
    [SerializeField] Button decreaseButton;

    [Header("Settings")]
    public int maxValue;
    public int minValue;

    public int Value 
    {
        get 
        { 
            int output = minValue;
            int.TryParse(input.text, out output);
            return output; 
        }
        set 
        { 
            if(value < minValue)
            {
                input.text = minValue.ToString();
            }
            else if (value > maxValue)
            {
                input.text = maxValue.ToString();
            }
            else
            {
                input.text = value.ToString(); 
            }
        }
    }

    public OnChangeEvent OnValueChanged
    {
        get { return input.onValueChanged; }
        protected set {}
    }    

    int lastValue;

    void Awake() 
    {
        input.contentType = ContentType.IntegerNumber;
    }
    
    void Start()
    {
        ValidateText();
        lastValue = int.Parse(input.text);
    }

    void OnEnable()
    {
        input.onValueChanged.AddListener(OnInputValueChanged);
        increaseButton.onClick.AddListener(Increase);
        decreaseButton.onClick.AddListener(Decrease);
    }
    
    void OnDisable() 
    {
        input.onValueChanged.RemoveListener(OnInputValueChanged);
        increaseButton.onClick.RemoveListener(Increase);
        decreaseButton.onClick.RemoveListener(Decrease);
    }

    bool ValidateText()
    {
        if(string.IsNullOrEmpty(input.text))
        {
            input.text = minValue.ToString();
            return false;
        }
        else return true;
    }

    void OnInputValueChanged(string newInput)
    {
        if(ValidateText())
        {
            Value = int.Parse(newInput);
        }
    }

    void Increase()
    {
        Value += 1;
    }

    void Decrease()
    {
        Value -= 1;
    }
}