Pretty clear cut
IMO, from the book, my emphasis
No matter how many stalkers you have, you are only allowed to nominate one enemy unit that is within line of sight of a Stalker. It doesn't stipulate that all Stalkers need to have
LoS, just one. But it does restrict the number of units.
If it said something like 'nominate a unit that is within
LoS of a Stalker from this formation' there would be more ambiguity. But as it states you can only nominate one unit.