Getting the Instance count for a device

What do you mean specifically by "instance".

LogicMonitor Instances are tied to checks within a device. So one device can have like 20 checks and each might have multiple instances or none. So a Windows device may have 1 instance for the nic check, 3 instances for drives, 20 instance for services, etc. for each DataSource, you can see the count of instances at the bottom of the Instances tab. For a combined count of all checks/instances a device had I'm not sure outside of checking each datasource and adding them up. Or write some code within LM or outside that counts it up for you.

Have a specific thing you are trying to do?