You can download the Intel AMT DTK source code, make the necessary changes, and recompile the software in order to lift the 20 computer limit.
1. Download the Intel AMT DTK source code
2. Extract source code from ZIP file to a working directory
3. Open the Manageability Commander Tool directory
4. Edit the MainForm.cs file
5. Change line 224 from:
if (Utils.GetNodeTagsofType(rootNode, typeof(AmtSystem)).Count >= 20)
if (Utils.GetNodeTagsofType(rootNode, typeof(AmtSystem)).Count >= 9999999)
6. Open the Manageability Commander Tool.sln Visual Studio solution (in the root of the source code folder)
Note: You can download Visual C# 2008 Express Edition for free if you don't have Visual Studio
7. Build the Manageability Commander Tool (in the Solution Explorer, right-click the project, and click Build)
8. Navigate to the Manageability Commander Tool\obj\debug folder and run your new executable
Let me know if this helps.