Installation on several PCs, some of them can't be activated.

Hello,
I have installed DSM on several PCs (4), some of them work, others cannot be activated. I've installed DSM on several PCs (4), some of them work, others can't be activated, hence my questions: how many times can I install DSM with a single account? Is it the large number of installations that is blocking? And how can I fix this?

Thanks