wrapping content in a StackPanel wpf

For me, a simple WrapPanel works just fine:

<WrapPanel Orientation="Horizontal" Width="500" />

Not inside a StackPanel or any other container. And setting Width to a constant value can be superior im some cases, because binding it to ActualWidth can prevent down-sizing (e.g. when parent control is down-sized, WrapPanel is not)

<StackPanel>
        <StackPanel.Style>
            <Style TargetType="{x:Type StackPanel}">
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type StackPanel}">
                            <WrapPanel/>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </StackPanel.Style>
    </StackPanel>

Create nested StackPanels which contain the required number of items.

In the example below, you have two rows, respectively occupied by the <StackPanel Orientation="Horizontal"> elements, which in turn each contain five items that will be displayed horizontally next to each other.

<StackPanel Orientation="Vertical">
    <StackPanel Orientation="Horizontal">
        <Item1 />
        <Item2 />
        <Item3 />
        <Item4 />
        <Item5 />
    </StackPanel>
    <StackPanel Orientation="Horizontal">
        <Item1 />
        <Item2 />
        <Item3 />
        <Item4 />
        <Item5 />
    </StackPanel>
</StackPanel>