You can use a Style applied to TabItem with a DataTrigger that will collapse it if the parent TabControl has only one item:
<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:sys="clr-namespace:System;assembly=mscorlib"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Grid.Resources>
<x:Array x:Key="tabData" Type="{x:Type sys:String}">
<sys:String>do</sys:String>
<sys:String>re</sys:String>
<sys:String>mi</sys:String>
</x:Array>
</Grid.Resources>
<TabControl ItemsSource="{StaticResource tabData}">
<TabControl.ItemContainerStyle>
<Style TargetType="{x:Type TabItem}">
<Style.Triggers>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TabControl}}, Path=Items.Count}" Value="1">
<Setter Property="Visibility" Value="Collapsed"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TabControl.ItemContainerStyle>
</TabControl>
</Grid>
If you want to get rid of the TabControl completely if there is only one item, that logic should probably be at a higher level.
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