Example of BadgeView with circle box using custom renderer - http://julianocustodio.com/badgeview

Badge.cs

using System;
using Xamarin.Forms;

namespace BadgeView
{
    public class Badge: AbsoluteLayout
    {

        protected Label Label;
        protected CircleBox Box;

        // Text property
        public static readonly BindableProperty TextProperty =
            BindableProperty.Create("Text", typeof(String), typeof(Badge), "");

        //Box color property
        public static readonly BindableProperty BoxColorProperty =
            BindableProperty.Create("BoxColor", typeof(Color), typeof(Badge), Color.Default);


        public Badge(double size, double fontSize)
        {
            HeightRequest = size;
            WidthRequest = size;

            // Box
            Box = new CircleBox
            {
                CornerRadius = HeightRequest/2
            };

            Box.SetBinding(BackgroundColorProperty, new Binding("BoxColor", source: this));
            Children.Add(Box, new Rectangle(0, 0, 1.0, 1.0), AbsoluteLayoutFlags.All);

            // Label
            Label = new Label
            {
                TextColor = Color.White,
                FontSize = fontSize,
                XAlign = TextAlignment.Center,
                YAlign = TextAlignment.Center
            };

            Label.SetBinding(Label.TextProperty, new Binding("Text",BindingMode.OneWay, source: this));

            Children.Add(Label, new Rectangle(0, 0, 1.0, 1.0), AbsoluteLayoutFlags.All);

            // Auto width
            SetBinding(WidthRequestProperty, new Binding("Text", BindingMode.OneWay, new BadgeWidthConverter(WidthRequest), source: this));

            // If not value = hide
            SetBinding(IsVisibleProperty, new Binding("Text", BindingMode.OneWay, new BadgeVisibleValueConverter(), source: this));

            // Default color
            BoxColor = Color.Red;
        }
        
        // Text
        public string Text
        {
            get { return (string)GetValue(TextProperty); }
            set { SetValue(TextProperty, value); }
        }
        
        // Color of the box
        public Color BoxColor
        {
            get { return (Color)GetValue(BoxColorProperty); }
            set { SetValue(BoxColorProperty, value); }
        }
        
    }
   
    class BadgeWidthConverter : IValueConverter
    {
        
        // Width ratio.
        const double widthRatio = 0.33;

        //Width of the base.
        readonly double baseWidth;

        public BadgeWidthConverter(double baseWidth)
        {
            this.baseWidth = baseWidth;
        }
        

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var text = value as string;
            if ((text != null) && (text.Length > 1))
            {
                return baseWidth * (1 + widthRatio * (text.Length - 1));
            }
            return baseWidth;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

    class BadgeVisibleValueConverter : IValueConverter
    {

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            var text = value as string;
            return !String.IsNullOrEmpty(text);
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
        
    }
}