You can reinstall windows on an SSD as often as you want. The OS if you are using an OEM version is tied to the motherboard, not the HDD or SSD. The retail version of an OS can be installed as many times as you want but must be uninstalled from the previous machine.
It depends what os you are going to install, as stated above for most versions of windows you get either oem which is locked to the first machine it is installed on, you also get full retail box set which you can move from one pc to another as long as it is installed in only one pc at a time.
The exception is windows 8, as microsoft in their endless need for greed, do not provide a full retail copy of windows 8 so no matter what version of windows 8 you install it will be fixed to that machine.
You don't tell it. It calculates what machine it is on when windows update runs (as well as when you activate). Basically it will compare the machine ID to the one on record and ensure that only 1 ID is getting activated at a time on that key.
That is leagally not up for debate. OEM windows is locked to a single motherboard. Retail is not.