You can either create one symbol by graphic (this thread may be helpful)or you can bind the Text property to the attribute so you need only one symbol for all graphics:
<Grid.Resources>
<esriSymbols:TextSymbol x:Key="LegendTextSymbol">
<esriSymbols:TextSymbol.ControlTemplate>
<ControlTemplate>
<Border Padding="10,5,10,5" Background="White" BorderBrush="Black">
<TextBlock
FontSize="{Binding Symbol.Size}"
Text="{Binding Attributes[NameOfTheAttributeUsedForLabeling]}"
Foreground="Blue"
FontFamily="{Binding Symbol.FontFamily}"
HorizontalAlignment="Center" />
</Border>
</ControlTemplate>
</esriSymbols:TextSymbol.ControlTemplate>
</esriSymbols:TextSymbol>
</Grid.Resources>