Reinstalling on the same system, even if you remove the whole FSX, doesn't count as an activation so, this can be done an unlimited number of times.
If you reformat Windows or change hardware you will consume one of your 6 activations.
If you REMEMBER to use the "Deactivate" a product with the Addon Manager BEFORE upgrading/reformatting your system, you won't consume an activation and this can be done an unlimited number of times too.
So, basically, you can always reinstall an unlimited number of times, even after a complete reformat or new hardware, as long as you don't forget there's a "Deactivate" function, that you don't have to use, if you want to "just" reinstall, without a reformat or a change of hardware.