MSP (Managed Service Provider) License is the best option for IT service providers. There's no limitation on the number of clients this license will be used with. Being a Service Provider you can install the purchased software at the clients side for inventory purposes for any time limit. There's no limitation on the number of PCs you can scan.

MSP license is same as Enterprise license, the only difference is that Enterprise license is limited to 1 company only, while MSP license allows you to install/use it on clients PCs.