Eldritch blast (PHB, pg. 237) says:
[...] Make a ranged spell attack against the target. On a hit, the target takes 1d10 force damage.
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level. You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.
So if I cast Eldritch Blast at a higher level, I must roll a separate attack roll for each beam. For example, if I'm a 5th level Warlock and I roll a 19 and a 20, I would only double the number of dice for the one beam it applied to, giving a total of 3d10, not 4d10.
Hex (PHB, pg. 251) says:
Until the spell ends, you deal an extra 1d6 necrotic damage to the target when you hit it with an attack.
Ordinarily, the way I understand this is: if I rolled a 19 and a 19 to hit with eldritch blast (again, assuming a 5th level Warlock), I would do a total of 2d10 + 1d6 damage (ignoring damage type for the moment, since I'm focusing more on the number of dice; also let's assume this Warlock doesn't have Agonizing Blast).
But what if I crit with eldritch blast? Under what circumstances do I double the number of dice for hex as well? Is this different if I were to roll a 19 and 20, critting with only one beam, or if I rolled 20 and 20 to crit with both beams?
I did wonder if it was the case that hex is unaffected by crits, such as if it were to be treated as secondary damage (like extra damage from saving throws, see: How does extra damage work for critical hits?), but Jeremy Crawford says hex is included in the crit, so if I crit with something like chill touch or some other warlock cantrip that involves a single attack roll, then the hex damage is clearly doubled. My confusion comes from having multiple attack rolls.
Just to restate the question: How is the additional damage from hex affected by an attack that has multiple attack rolls, some or all of which may be critical hits? Does eldritch blast's multiple attack rolls effectively increase the likelihood of getting to double the damage from hex via a critical hit?