It's better to speak of simultaneous activations than of PCs to get behind how this system works.
You are allowed to have at most 3 simultaneous activations of SAM.
If you install SAM and activate it on 3 separate machines and then deactivate the license on one computer, you can go ahead and activate the same license on computer number 4, because you only have 3 simultaneous activations. You can also add computer no. 5 and 6 in trial mode without getting in conflict with your license.
The only problem is when you destroy/remove/corrupt/replace the harddrive/ssd/disk-volume of an activated computer and replace it/can't use it anymore. In that case, SAM isn't uninstalled and the license isn't deactivated, therefore counting towards the simultaneous activation limit, despite not being in use anymore. In such a case you need to talk to the support people at Spacial and have them deactivate your license. They will only ever deactivate the oldest activation as they have no way of telling the separate activations apart. They won't reset ALL of your activations at once. So if for some reason the newest activated computer fails and needs to be replaced, you'll end up having to get the activations reset on all of your machines, one at a time.
Hope this helps more than it confuses.
If you need any specific help with activations, you'll have to refer to the official helpdesk at https://support.spacial.com