You must target six different creatures. If you can't, you can't cast Hex. If some of the creatures become illegal targets before the spell resolves, Hex will still destroy the rest of them.

Hex : règles, fonctionnement et interactions (MTG)

Hex is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Destroy six target creatures.

How it works : Hex

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2005· Wizards of the Coast
You must target six different creatures. If you can't, you can't cast Hex. If some of the creatures become illegal targets before the spell resolves, Hex will still destroy the rest of them.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Hex.

Swiftfoot Boots

Equipped creature has hexproof and haste. (It can't be the target of spells or abilities your opponents control. It can attack and {T} no matter when it came under your control.) Equip {1} ({1}: Attach to target creature you control. Equip only as a sorcery.)

Heroic Intervention

Permanents you control gain hexproof and indestructible until end of turn.

Shadowspear

Equipped creature gets +1/+1 and has trample and lifelink. {1}: Permanents your opponents control lose hexproof and indestructible until end of turn. Equip {2}

Dawn's Truce

Gift a card (You may promise an opponent a gift as you cast this spell. If you do, they draw a card before its other effects.) You and permanents you control gain hexproof until end of turn. If the gift was promised, permanents you control also gain indestructible until end of turn.

Snakeskin Veil

Put a +1/+1 counter on target creature you control. It gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Tamiyo's Safekeeping

Target permanent you control gains hexproof and indestructible until end of turn. You gain 2 life. (A permanent with hexproof and indestructible can't be the target of spells or abilities your opponents control. Damage and effects that say "destroy" don't destroy it.)

Plaza of Heroes

{T}: Add {C}. {T}: Add one mana of any color. Spend this mana only to cast a legendary spell. {T}: Add one mana of any color among legendary permanents you control. {3}, {T}, Exile this land: Target legendary creature gains hexproof and indestructible until end of turn.

Veil of Summer

Draw a card if an opponent has cast a blue or black spell this turn. Spells you control can't be countered this turn. You and permanents you control gain hexproof from blue and from black until end of turn. (You and they can't be the targets of blue or black spells or abilities your opponents control.)

Tyvar's Stand

Target creature you control gets +X/+X and gains hexproof and indestructible until end of turn. (It can't be the target of spells or abilities your opponents control. Damage and effects that say "destroy" don't destroy it.)

Champion's Helm

Equipped creature gets +2/+2. As long as equipped creature is legendary, it has hexproof. (It can't be the target of spells or abilities your opponents control.) Equip {1}

Flare of Fortitude

You may sacrifice a nontoken white creature rather than pay this spell's mana cost. Until end of turn, your life total can't change, and permanents you control gain hexproof and indestructible.

Collective Resistance

Escalate {G} (Pay this cost for each mode chosen beyond the first.) Choose one or more — • Destroy target artifact. • Destroy target enchantment. • Target creature gains hexproof and indestructible until end of turn.