Wrapping is the default behavior for tab controls. Any style or control template you're using that interferes with that?
The following code
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Window1" Height="300" Width="300" AllowsTransparency="True" WindowStyle="None">
<Grid>
<TabControl>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
<TabItem Header="TabItem"/>
</TabControl>
</Grid>
</Window>
yields