Let's imagine the situation
I have Traktor Control S3, use it with Traktor Pro3/Pro4 on 2 devices with one serial number simultaneously.
Maybe one of these devices will break in 2-3 years.
Can I install it on new second device ?
Yes, that should be fine. I never heard about anyone being limited by installs. I now there is officially a 2 device limit, but I have yet not hear anyone running into it.