Ahpingko
the program can be activated with same serial (or activated code) multiple time on multiple PC. the only limitation of the series are locked option/feature of the software, like studio matching, or custom brightness/whitebalance.
and for spyder2,3 express model, it only allowed calibration of 1 display(technically is running of single profile), or means multiple monitor will use same one profile. (but not sure with spyder5, heard they lifted this limitation)
colorimeter is to tune the computer actually, not the monitor. monitor are more of a fixed physical output. monitor display rely on the signal or data (analog for VGA port, digital for DVI/HDMI/DP port) from the graphic processing unit, which base on color profiles window/driver has given.
after tuning, it suppose to show accurate color of what you are suppose to see at the limitation of the monitor physical ability. so your edited photo should be as neutral base on your editing.
eg if your computer color has run off toward more pink tint, you might adjust your photo into the green zone without knowing it, and generally we calibrated monitor will see your photo in green tint.
calibrated monitor are good. because generally, iphone and smartphones are build to be as close to whitebalanced as possible, while laptop and lcd are a little colder, and crt (if there are any around) gets warmer overtime.
so if you edited the photo on your pc, dont worry what kind of result your friends see in their PC, because their PC/laptop might not be calibrated and their color might or may not be off, but rest assure, their mobile are the closest neutral color to your computer color.
also if you are not sure what editing you did, might gone wrong or not is whole another issue of your own color perception.
another thing that you will know your friend computer color has gone wrong or not is, when they upload their own photo, the whole/several album are all tinted towards a color... which i see commonly in many photographer's fb, especially towards green tint due to their computer(laptop) are too blueish red...
ok, i finished my coffee, back to work.

thanks for the comments.. i tuned my macbook and IPS display.. they now looked warmer.. lol