I originally read this related question as wanting to find out how to have a party with access to every spell during an adventuring day with as few members as possible. It turned out that the linked question was asking something different, but I am also curious about my original interpretation.

How few characters are needed so that at the end of a long rest those characters will be able to prepare their spells so that any spell may be cast at least once?

• Use only the rules in the Player's Handbook


• PCs may be of any level


• Multiclassing and feats are permitted


• The party must be able to cast any spell on a given day (but not necessarily every spell) from either a class feature, racial trait, or feat.

The minimum number of character is six

The Party

Our party consists of:

• Onus the wizard 17


• Twofa the bard (College of Lore) 18


• Threen the cleric (Tempest Domain) 17


• Fourp the bard 18/warlock 2


• Fivet the bard 10/druid (Circle of the Land) 9


• Sixka the ranger 9/paladin 9

The Spells

To start off, the wizard feature Spellbook makes any wizard spells attainable during the wizard's preparation. We also have a level 17 cleric who can prepare any cleric spell. From here, we will work backwards through the levels to show that all spells are accounted for:

Level 9

There are two 9th level spells that clerics and wizards together don't get access to:

• Storm of Vengeance (covered by Twofa's first level 18 Magical Secrets)


• Power Word: Heal (covered by Twofa's bard Spells Known)

Level 8

There are three 8th level spells that clerics and wizards together don't get access to:

• Tsunami (covered by Twofa's second level 18 Magical Secrets)


• Glibness (covered by Twofa's bard Spells Known)


• Animal Shapes (covered by Fourp's first level 18 Magical Secrets)

There are zero 7th level spells that clerics and wizards don't get access to, so on to:

Level 6

There are four 6th level spells that clerics and wizards together don't get access to:

• Wind Walk (covered by Fourp's second level 18 Magical Secrets)


• Wall of Thorns (covered by Twofa's first level 14 Magical Secrets)


• Transport Via Plants (covered by Twofa's second level 14 Magical Secrets)


• Conjure Fey (covered by Fourp's first level 14 Magical Secrets)

For spells lower than level 6, Fivet can cover any druid spell through preparation, so:

Level 5

There are six 5th level spells that clerics, druids, and wizards together don't get access to:

• Swift Quiver (covered by Fourp's second level 14 Magical Secrets)


• Destructive Wave (covered as one of Threen's domain spells)


• Conjure Volley (covered by Twofa's first level 10 Magical Secrets)


• Circle of Power (covered by Twofa's second level 10 Magical Secrets)


• Banishing Smite (covered by Fourp's first level 10 Magical Secrets)


• Antilife Shell (covered by Twofa's bard Spells Known)

Level 4

There are four 4th level spells that clerics, druids, and wizards together don't get access to:

• Staggering Smite (covered by Fourp's second level 10 Magical Secrets)


• Aura of Purity (covered by Fivet's first level 10 Magical Secrets)


• Aura of Life (covered by Fivet's second level 10 Magical Secrets)


• Compulsion (covered by Twofa's bard Spells Known)

For spells lower than level 3, Sixka can cover any paladin spell through preparation, so:

Level 3

There are three 3rd level spells that only bards, rangers, sorcerers, and warlocks together get access to:

• Lightning Arrow (covered by Sixka's ranger Spells Known)


• Hunger of Hadar (covered by Twofa's Additional Magical Secrets)


• Conjure Barrage (covered by Sixka's ranger Spells Known)

Level 2

There are two 2nd level spells that only bards, rangers sorcerers, and warlocks together get access to:

• Enthrall (covered by Twofa's bard Spells Known)


• Cordon of Arrows (covered by Sixka's ranger Spells Known)

Level 1

There are eight 1st level spells that only bards, rangers, sorcerers, and warlocks together get access to:

• Hex (covered by Twofa's Additional Magical Secrets)


• Hellish Rebuke (covered by Fourp's warlock Spells Known)


• Hail of Thorns (covered by Sixka's ranger Spells Known)


• Hunter's Mark (covered by Sixka's ranger Spells Known)


• Ensnaring Strike (covered by Sixka's ranger Spells Known)


• Dissonant Whispers (covered by Twofa's Spells Known)


• Arms of Hadar (covered by Fourp's warlock Spells Known)


• Armor of Agathys (covered by Fourp's warlock Spells Known)

Cantrips

For cantrips, I will go through each character's cantrips to show they are all covered

• Onus (5 wizard cantrips)
  
  
  • Acid Splash
  
  
  • Chill Touch
  
  
  • Fire Bolt
  
  
  • Shocking Grasp
  
  
  • Ray of Frost
  
  


• Twofa (4 bard cantrips)
  
  
  • Blade Ward
  
  
  • Dancing Lights
  
  
  • Friends
  
  
  • Vicious Mockery
  
  


• Threen (5 cleric cantrips)
  
  
  • Guidance
  
  
  • Resistance
  
  
  • Sacred Flame
  
  
  • Spare the Dying
  
  
  • Thaumaturgy
  
  


• Fourp (4 bard cantrips, 2 warlock cantrips)
  
  
  • Light
  
  
  • Mage Hand
  
  
  • Mending
  
  
  • Message
  
  
  • Eldritch Blast
  
  
  • Poison Spray
  
  


• Fivet (4 bard cantrips, 4 druid cantrips)
  
  
  • Minor Illusion
  
  
  • Prestidigitation
  
  
  • True Strike
  
  
  • (any bard cantrip)
  
  
  • Druidcraft
  
  
  • Produce Flame
  
  
  • Shillelagh
  
  
  • Thorn Whip