You got the Fel-PRO MS98014T lower intake manifold kit right?
If that is the kit he got then they only give you six o-rings because they are for the lower tip of the injectors, I am pretty sure you do not get 12 of them because most do not totally remove the injectors from the fuel rail when doing the LIM job, of course the o-rings will also fit the top or bottom as they are the same size.
I was able to replace mine without doing anything more than lubricating them with some clean synthetic oil, or non synth if that is what you have, try to get one half under the lip of where it seats on the injector, and then roll the other side on, do not worry about stretching the o-ring a tad while rolling it on, as long as it does not stay stretched too long it should be fine.
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA