First, separate ISO files are not the point of focus, but separate license is. An ISO image can be used to install the OS in numerous computers, but a license can activate only one of those installations at a time.
Second, as windows 7 is a qualified previous OS, the digital entitlement is granted via an upgradation. You need to upgrade it first. The process is ....
Upgrade > Acquire Digital Entitlement > Go for how many clean installs you want, without using a key. If it is not upgraded at the first place but a direct clean installation of windows 10 is done, you must have acquired new windows 10 license keys from microsoft for all those computers.
Third, if you want to upgrade to windows 10 from any windows 7 device, do it within the cut-off date, not after that.
If you upgrade within the due date, it will be a free upgrade; but after that you have to pay for the windows 10 license.