I hope that i can find an answer here.

I had tried both InputNumber from Microsoft .NET or even NumberInput from Blazor Bootstrap.

<NumberInput
                                                TValue="int"
                                                @bind-Value="@_quantity"
                                                @onchange="@(e => _quantity = (int)e.Value)"
                                                EnableMinMax="true"
                                                Min="1"
                                                Max="300"
                                                Class="bg-gray-50 border border-gray-300 text-gray-900 text-sm rounded-lg focus:ring-primary-600 focus:border-primary-600 block w-full p-2.5 dark:bg-gray-700 dark:border-gray-600 dark:placeholder-gray-400 dark:text-white dark:focus:ring-primary-500 dark:focus:border-primary-500"
                                                Placeholder="1"
                                            />
                                            @* <InputNumber *@
                                            @*     TValue="int" *@
                                            @*     @bind-Value="OrderItemRequestInput.Quantity" *@
                                            @*     name="Quantity" *@
                                            @*     id="Quantity" *@
                                            @*     class="bg-gray-50 border border-gray-300 text-gray-900 text-sm rounded-lg focus:ring-primary-600 focus:border-primary-600 block w-full p-2.5 dark:bg-gray-700 dark:border-gray-600 dark:placeholder-gray-400 dark:text-white dark:focus:ring-primary-500 dark:focus:border-primary-500" *@
                                            @*     placeholder="1" *@
                                            @*     required/> *@

private PagedResult<SpecificOrderResponse> _orders;
    [Parameter]
    public PagedResult<SpecificOrderResponse> Orders
    {
        get => _orders;
        set
        {
            _orders = value;
            if (_orders?.Results?.Any() == true)
            {
                OrderItemRequestInput.OrderId = _orders.Results.First().Id;
            }
        }
    }

    [SupplyParameterFromForm] private CreateOrderItemRequest OrderItemRequestInput { get; set; } = new();

 private async Task AddOrderItem(EditContext editContext)
    {
        
        if (!editContext.Validate())
        {
            return;
        }
        
        Logger.LogInformation($"_Quantity Input: {_quantity}");

        OrderItemRequestInput.Quantity = _quantity;
        
        Logger.LogInformation($"Quantity Input: {OrderItemRequestInput.Quantity}");

        OrderItemRequestInput.UnitId = SelectedUnitId.Value;
        OrderItemRequestInput.ProductId = Product.ID;
        OrderItemRequestInput.ResellerPrice = (decimal)CustomerPriceExGST;
        
        Logger.LogInformation($"Order id {OrderItemRequestInput.OrderId} is selected");

        Logger.LogInformation($"Add {OrderItemRequestInput.Quantity} product: {OrderItemRequestInput.ProductId} to order: {OrderItemRequestInput.OrderId}");
        

        if (LnkOrderItemService == null)
        {
            Logger.LogError("LnkOrderItemService is null");
            return;
        }

        if (HttpContextAccessor == null || HttpContextAccessor.HttpContext == null)
        {
            Logger.LogError("HttpContextAccessor or HttpContextAccessor.HttpContext is null");
            return;
        }
        
        try
        {
            OrderItemResponse = await LnkOrderItemService.AddOrderItemIntoOrder(HttpContextAccessor.HttpContext, OrderItemRequestInput);
            Console.WriteLine("Result Message: " + OrderItemResponse.Message);
        }
        catch (Exception ex)
        {
            Logger.LogError($"Add {OrderItemRequestInput.Quantity} product: {OrderItemRequestInput.ProductId} with unit: {OrderItemRequestInput.UnitId} to order: {OrderItemRequestInput.OrderId} has error {ex.StackTrace}");
        }
    }

On the UI, i have tried to put 2 or 3 or even 300. However, the OrderLineItemRequestInput.Quantity is always 1

Is there anything i do wrong?

I try to get input of Quantity from EditForm above.