public ObservableCollection<NumberViewModel> Nummer { get; private set; } = new ObservableCollection<NumberViewModel>();
    private int _currentNumber;
    public int current
    {
        get
        {
            return _currentNumber;
        }
        set
        {
            _currentNumber = value;
            OnPropertyChanged();
        }
    }

    public ICommand CountUpCommand { get; private set; }
    public ICommand DelCommand { get; private set; }

    Number Zahl = new Number();

    public CounterViewModel()
    {
        CountUpCommand = new Command(CountUp);
        DelCommand = new Command(SetZero);
    }

    public void SetZero()
    {
        current = 0;
        Nummer.Add(new NumberViewModel { Num = current});
    }

    public void CountUp()
    {

            current = current + 1;
            Nummer.Add(new NumberViewModel { Num = current });

    }

<StackLayout>
        <Button Text="+1" Command="{Binding CountUpCommand}" x:Name="plusOne" />
        <Button Text="DEL" Command="{Binding DelCommand}" />
    </StackLayout>
<Label Text="--------------" />
<StackLayout>
    <ListView ItemsSource="{Binding Nummer}">
        <ListView.ItemTemplate>
            <DataTemplate>
                <TextCell
                Text="{Binding Num}"
                x:Name="ElNr"
                />
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>