ListViewAutoSizeBehavior.cs

namespace Xamarin.Forms
{
  public class AutoSizeBehavior : Behavior<ListView>
  {
    ListView _ListView;

    protected override void OnAttachedTo(ListView bindable)
    {
      _ListView = bindable;
      bindable.HeightRequest = 0;
      bindable.ItemAppearing += AppearanceChanged;
      bindable.ItemDisappearing += AppearanceChanged;
    }

    protected override void OnDetachingFrom(ListView bindable)
    {
      bindable.ItemAppearing -= AppearanceChanged;
      bindable.ItemDisappearing -= AppearanceChanged;
      _ListView = null;
    }

    private void AppearanceChanged(object sender, ItemVisibilityEventArgs e) =>
      UpdateHeight(e.Item);

    void UpdateHeight(object item)
    {
      double height;
      if (_ListView.HasUnevenRows)
      {
        if ((height = _ListView.HeightRequest) == (double)VisualElement.HeightRequestProperty.DefaultValue)
          height = 0;

        height += MeasureRowHeight(item);
      }
      else
      {
        height = _ListView.RowHeight;
        if (height == (int)ListView.RowHeightProperty.DefaultValue)
          _ListView.RowHeight = (int)(height = MeasureRowHeight(item));
      }
      SetHeight(height);
    }

    double MeasureRowHeight(object item)
    {
      var template = _ListView.ItemTemplate;
      var cell = (Cell)template.CreateContent();
      cell.BindingContext = item;

      // TODO: redundant
      cell.ForceUpdateSize();

      var height = cell.RenderHeight;
      var mod = height % 1;
      if (mod > 0)
        height = height - mod + 1;
      return height;
    }

    void SetHeight(double rowHeight)
    {
      var height = 0d;
      // TODO if header or footer is string etc.
      if (_ListView.Header is VisualElement header)
        height += header.Height;
      if (_ListView.Footer is VisualElement footer)
        height += footer.Height;

      var tiv = (ITemplatedItemsView<Cell>)_ListView;
      height += tiv.TemplatedItems.Count * rowHeight;

      _ListView.HeightRequest = height;
    }
  }
}

why to check for _ListView.HasUnevenRows just do this
void UpdateHeight(object item)
{
double height;
if ((height = _ListView.HeightRequest) == (double)VisualElement.HeightRequestProperty.DefaultValue)
height = 0;
height += MeasureRowHeight(item);
SetHeight(height);
}
it will work fine with number of rows with height set for viewcell