English major time!
Here's the statement we're dealing with: "Any Unit, friendly or enemy, under this line takes a number of hits equal to the number of models in the unit under the line."
Sadly, the phrase "under the line" is ambiguous as to what noun it modifies; there's some words left out. Compare:
Any Unit, friendly or enemy, under this line takes a number of hits equal to the number of models in the unit that are under the line.
or
Any Unit, friendly or enemy, under this line takes a number of hits equal to the number of models in the unit that is under the line.
Without those clarifying words, the sentence means neither and both equally; we don't know if "under the line" refers to the unit or the models. However, there are two points to consider:
1. The sentence already specifies that the unit is under the line (after "friendly or enemy"), so to state it again at the end would be redundant. That's a reasonable indicator that "under the line" refers to the models.
2. It's supremely unlikely that GW intends for Necrons to be tossing around the equivalent of apocalypse templates.
As such, if this comes up at the Doubles event, expect me to rule that the number of models under the line determines the number of hits. So, don't spam Death Rays and expect to just flat-out win in turn one.