The thread title explains how this works: basically, you can have 8 veterans from each Smash Bros. game and 25% of the stages introduced in each Smash Bros. game (rounding to the closest integer, Smash for 3DS and Smash for Wii U count as separate games for the stages).
Let me show you my take:
Smash 64 characters:![Mario :ultmario: :ultmario:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Donkey Kong :ultdk: :ultdk:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Link :ultlink: :ultlink:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Samus :ultsamus: :ultsamus:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Yoshi :ultyoshi: :ultyoshi:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Kirby :ultkirby: :ultkirby:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Pikachu :ultpikachu: :ultpikachu:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Luigi :ultluigi: :ultluigi:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Smash 64 stages (2): Hyrule Castle, Mushroom Kingdom
Melee characters:![Peach :ultpeach: :ultpeach:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Bowser :ultbowser: :ultbowser:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ice Climbers :ulticeclimbers: :ulticeclimbers:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Zelda :ultzelda: :ultzelda:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Pichu :ultpichu: :ultpichu:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Marth :ultmarth: :ultmarth:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ganondorf :ultganondorf: :ultganondorf:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Mewtwo :ultmewtwo: :ultmewtwo:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Melee stages (6, Melee is a weird case as it has 26 stages and 26/4=6.5): Fountain of Dreams, Temple, Great Bay, Princess Peach's Castle, Kongo Falls, Pokémon Stadium.
Brawl characters:![Pit :ultpit: :ultpit:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Wario :ultwario: :ultwario:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Pokemon Trainer :ultpokemontrainer: :ultpokemontrainer:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Diddy Kong :ultdiddy: :ultdiddy:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Sonic :ultsonic: :ultsonic:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![King Dedede :ultkingdedede: :ultkingdedede:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Olimar :ultolimar: :ultolimar:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Toon Link :ulttoonlink: :ulttoonlink:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Brawl stages (8):![Delfino Plaza :delfino: :delfino:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Spear Pillar :spearpillar: :spearpillar:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Pirate Ship :pirateship: :pirateship:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Green Hill Zone :greenhill: :greenhill:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Hanenbow :hanenbow: :hanenbow:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Frigate Orpheon :frigate: :frigate:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Castle Siege :castlesiege: :castlesiege:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Smashville :smashville: :smashville:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Smash 4 characters:![Villager :ultvillager: :ultvillager:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Wii Fit Trainer :ultwiifittrainer: :ultwiifittrainer:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Little Mac :ultlittlemac: :ultlittlemac:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Palutena :ultpalutena: :ultpalutena:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Shulk :ultshulk: :ultshulk:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Bowser Jr. :ultbowserjr: :ultbowserjr:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Corrin :ultcorrin: :ultcorrin:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Bayonetta :ultbayonetta: :ultbayonetta:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Smash for 3DS stages (8): Paper Mario, Unova Pokémon League, Magicant, Arena Ferox, Reset Bomb Forest, Tortimer Island, Boxing Ring, Gaur Plain
Smash for Wii U stages (8, Smash for Wii U is in a situation like Melee, where it has 34 stages and 34/4=8.5): Mushroom Kingdom U, Super Mario Maker, Skyloft, Jungle Hijinxs, Pyrosphere, Town and City, Wii Fit Studio, Umbra Clock Tower
Ultimate characters:![Dark Samus :ultdarksamus: :ultdarksamus:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Chrom :ultchrom: :ultchrom:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Inkling :ultinkling: :ultinkling:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Ridley :ultridley: :ultridley:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![King K. Rool :ultkrool: :ultkrool:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Isabelle :ultisabelle: :ultisabelle:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Incineroar :ultincineroar: :ultincineroar:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Joker :ultjoker: :ultjoker:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Ultimate stages (2): Moray Towers, Mementos
Let me show you my take:
Smash 64 characters:
Smash 64 stages (2): Hyrule Castle, Mushroom Kingdom
Melee characters:
Melee stages (6, Melee is a weird case as it has 26 stages and 26/4=6.5): Fountain of Dreams, Temple, Great Bay, Princess Peach's Castle, Kongo Falls, Pokémon Stadium.
Brawl characters:
Brawl stages (8):
Smash 4 characters:
Smash for 3DS stages (8): Paper Mario, Unova Pokémon League, Magicant, Arena Ferox, Reset Bomb Forest, Tortimer Island, Boxing Ring, Gaur Plain
Smash for Wii U stages (8, Smash for Wii U is in a situation like Melee, where it has 34 stages and 34/4=8.5): Mushroom Kingdom U, Super Mario Maker, Skyloft, Jungle Hijinxs, Pyrosphere, Town and City, Wii Fit Studio, Umbra Clock Tower
Ultimate characters:
Ultimate stages (2): Moray Towers, Mementos