I don't think that is something we are really designed to do.
We have UDD that scans devices.
Do you mean "How many machines are online right now?"
If so, you basically have to hit every machine some how. A custom script, batch file task, etc...
Or do you mean "How many machines were online last monday?"
If so, there may be enough data in inventory, such as last boot time, uptime, etc that would allow a Business Intelligence tool to determine this information.
To be more clear ...
We just want to know how many machines are online right now?
And the machines has LANDesk agent installed.