An MSBuild targets file to retrieve the Team Build Number for TFS 2012. Just <Import> this targets file and specify DependsOnTargets="TeamBuildNumber" in your <Target>.

TeamBuildNumber.targets

<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

  <Target Name="TeamBuildNumber">

    <GetTeamBuildNumber TeamFoundationServerUrl="$(TeamFoundationServerUrl)" BuildUri="$(BuildUri)" ContinueOnError="true">
      <Output TaskParameter="TeamBuildNumber" PropertyName="TF_BUILD_BUILDNUMBER" />
    </GetTeamBuildNumber>
    
  </Target>
  
  <PropertyGroup>
    <TeamFoundationApiVersion>11.0.0.0</TeamFoundationApiVersion><!-- 11 = TFS 2012 -->
  </PropertyGroup>

  <UsingTask TaskName="GetTeamBuildNumber" 
             TaskFactory="CodeTaskFactory" 
             AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v$(MSBuildToolsVersion).dll">
    <ParameterGroup>
      <TeamFoundationServerUrl ParameterType="System.String" Required="true" />
      <BuildUri ParameterType="System.String" Required="true" />
      <TeamBuildNumber ParameterType="System.String" Output="true" />
    </ParameterGroup>
    <Task>
      <Reference Include="Microsoft.TeamFoundation.Client, Version=$(TeamFoundationApiVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
      <Reference Include="Microsoft.TeamFoundation.Build.Client, Version=$(TeamFoundationApiVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
      <Using Namespace="Microsoft.TeamFoundation.Client" />
      <Using Namespace="Microsoft.TeamFoundation.Build.Client" />
      <Code Type="Fragment" Language="cs">
        <![CDATA[
TeamBuildNumber = "";
if (string.IsNullOrEmpty(TeamFoundationServerUrl) || string.IsNullOrEmpty(BuildUri)) {
  Log.LogMessage("TeamFoundationServerUrl or BuildUri not specified.");
  return false;
}
var collection = TfsTeamProjectCollectionFactory.GetTeamProjectCollection(new Uri(TeamFoundationServerUrl));  
var buildServer = collection.GetService<IBuildServer>();  
var buildDetail = buildServer.GetBuild(new Uri(BuildUri));
TeamBuildNumber = buildDetail.BuildNumber;
        ]]>
      </Code>
    </Task>
  </UsingTask>

</Project>