Shell Smash is learned by 23 species by level and 4 by egg

Effect, in PokéAPI's words

“Raises user’s Attack, Special Attack, and Speed by two stages. Lower user’s Defense and Special Defense by one stage.”

Who learns it

27 species in all, 8 of 10 games shown

Scarlet and Violet: 14 species

Species	Method	Level
Magcargo	Level up	Evolve
Cloyster	Level up	1
Squirtle	Level up	27
Wartortle	Level up	35
Blastoise	Level up	35
Shellder	Level up	44
Minior	Level up	45
Torkoal	Level up	60
Sinistea	Level up	60
Polteageist	Level up	60
Turtwig	Egg
Grotle	Egg
Torterra	Egg
Chewtle	Egg

Brilliant Diamond and Shining Pearl: 13 species

Species	Method	Level
Magcargo	Level up	Evolve, 1
Cloyster	Level up	1
Huntail	Level up	1
Gorebyss	Level up	1
Squirtle	Level up	27
Wartortle	Level up	35
Blastoise	Level up	35
Shellder	Level up	44
Clamperl	Level up	50
Omanyte	Level up	55
Torkoal	Level up	60
Omastar	Level up	63
Shuckle	Level up	65

Sword and Shield: 18 species

Species	Method	Level
Cloyster	Level up	1
Squirtle	Level up	27
Wartortle	Level up	35
Blastoise	Level up	35
Binacle	Level up	40
Barbaracle	Level up	42
Shellder	Level up	44
Dwebble	Level up	44
Turtonator	Level up	44
Tirtouga	Level up	45
Crustle	Level up	50
Carracosta	Level up	51
Omanyte	Level up	55
Torkoal	Level up	60
Sinistea	Level up	60
Polteageist	Level up	60
Omastar	Level up	63
Shuckle	Level up	65

Let's Go Pikachu and Let's Go Eevee: 3 species

Species	Method	Level
Shellder	Level up	40
Omanyte	Level up	48
Omastar	Level up	60

Ultra Sun and Ultra Moon: 16 species

Species	Method	Level
Magcargo	Level up	Evolve, 1
Cloyster	Level up	1
Crustle	Level up	1, 43
Binacle	Level up	1
Barbaracle	Level up	1
Shuckle	Level up	34
Dwebble	Level up	37
Turtonator	Level up	37
Tirtouga	Level up	38
Carracosta	Level up	40
Minior	Level up	45
Torkoal	Level up	47
Omanyte	Level up	50
Clamperl	Level up	50
Shellder	Level up	56
Omastar	Level up	67

Sun and Moon: 16 species

Species	Method	Level
Magcargo	Level up	Evolve, 1
Cloyster	Level up	1
Crustle	Level up	1, 43
Binacle	Level up	1
Barbaracle	Level up	1
Shuckle	Level up	34
Dwebble	Level up	37
Turtonator	Level up	37
Tirtouga	Level up	38
Carracosta	Level up	40
Minior	Level up	45
Torkoal	Level up	47
Omanyte	Level up	50
Clamperl	Level up	50
Shellder	Level up	56
Omastar	Level up	67

Omega Ruby and Alpha Sapphire: 14 species

Species	Method	Level
Cloyster	Level up	1
Crustle	Level up	1, 43
Binacle	Level up	1
Barbaracle	Level up	1
Shuckle	Level up	34
Dwebble	Level up	37
Magcargo	Level up	38
Tirtouga	Level up	38
Carracosta	Level up	40
Torkoal	Level up	47
Omanyte	Level up	50
Clamperl	Level up	50
Shellder	Level up	56
Omastar	Level up	67

X and Y: 14 species

Species	Method	Level
Cloyster	Level up	1
Torkoal	Level up	1, 65
Crustle	Level up	1, 43
Binacle	Level up	1
Barbaracle	Level up	1
Shuckle	Level up	34
Dwebble	Level up	37
Magcargo	Level up	38
Tirtouga	Level up	38
Carracosta	Level up	40
Omanyte	Level up	50
Clamperl	Level up	50
Shellder	Level up	56
Omastar	Level up	67