This is a Windows limitation, but you can remove this limitation by setting this registry value:
Key: HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer
Name: MultipleInvokePromptMinimum
Type: DWORD

Set the value to 16 (decimal) to allow for an unlimited number of files to be selected and show the "Hash with HashTools" context menu item. You can read more about this registry setting here:
http://support.microsoft.com/kb/2022295