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
  
  

You need three casters: A wizard 17, a bard 18 and either a druid 17, a cleric 17 or another bard 18 (your choice). First, the wizard can get all the spells on his list. This leaves 4 9th level spells out. The first bard handles three of these: Power Word Heal (The Bard Takes as a Bard Spell Known at 17), Storm of Vengeance (the first 18th level Magic Secret) and Mass Heal (the second 18th level Magic Secret). That leaves True Resurrection, which can be supplied by a druid 17, a cleric 17 or a bard 18 (with his 18th level Magic Secret).

Why did I only focus on 9th level spells? Because the wizard can supply Wish, and Wish can supply any 8th level or lower spell desired.