Generally, Microsoft's licence terms apply. These state that the product key is valid for use on one device. To try to circumvent this is against the law, according to Microsoft.
The product keys are therefore designed for use on one computer, so you need a product key for each device on which you would like to use the software to avoid violating Microsoft's licence terms.
Programms, that expressly indicate for how many devices they are intended, are exempt from this rule.
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