How many temporary hit points do you receive when you cast the spell?
1d10 + caster level.
Is "number of temporary hit points" a random variable numeric effect of the spell? Certainly - for a 10th level caster, you receive a random number between 11 and 20.
Therefore, "number of temporary hit points" qualifies for the multiplier from Empower.
If the spell stated "You receive a number of temporary hit points equal to your caster level (max 10). In addition, you also receive 1d10 temporary hit points. These effects stack." ... or something like that, then there are two numeric effects, only one of which is variable.
But the way it is phrased, there is one numeric effect, and it is variable.
-Hyp.