Xjyuk

The wight has an attack called Life Drain:

Life Drain. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 5 (1d6 + 2) necrotic damage. The target must succeed on a DC 13 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

A humanoid slain by this attack rises 24 hours later as a zombie under the wight's control, unless the humanoid is restored to life or its body is destroyed. The wight can have no more than twelve zombies under its control at one time.

If the same wight hits a character with it multiple times, or a number of wights hit the same character with it, assuming the character fails their Constitution save each time, do the max HP reduction effects stack?

I know magical effects don't normally stack, but this isn't listed as being magical. Additionally, if it doesn't stack, I can't see how most characters could ever have their max HP reduced to 0.

It looks like the answer is: No it doesn't stack

While this may be a magical effect, the combining of magical effects you are referencing refers to spells specifically.

Combining Magical Effects

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect-such as the highest bonus-from those castings applies while their durations overlap.

For example, if two clerics cast bless on the same target, that character gains the spell's benefit only once; he or she doesn't get to roll two bonus dice.

(Player's Handbook, page 205)

Bolded by me.

As you can see this rule is referencing spells.

However, there is also this errata in the DMG:

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook.

The Life Drain ability which wights possess is not a spell, but rather a creature's ability. But apparently in recent errata it doesn't stack.