4 species have Swarm as a hidden ability, 27 as a normal one

Effect, in PokéAPI's words

“Strengthens Bug moves to inflict 1.5× damage at 1/3 max HP or less.”

As a hidden ability

Species	Slot	Note
Joltik	Hidden
Galvantula	Hidden
Larvesta	Hidden
Volcarona	Hidden

As a normal ability

Species	Slot	Note
Beedrill	Normal
Scyther	Normal
Ledyba	Normal
Ledian	Normal
Spinarak	Normal
Ariados	Normal
Scizor	Normal
Heracross	Normal
Beautifly	Normal
Volbeat	Normal
Kricketune	Normal
Mothim	Normal
Sewaddle	Normal
Leavanny	Normal
Venipede	Normal
Whirlipede	Normal
Scolipede	Normal
Karrablast	Normal
Escavalier	Normal
Durant	Normal
Grubbin	Normal
Blipbug	Normal
Dottler	Normal
Orbeetle	Normal
Kleavor	Normal
Nymble	Normal
Lokix	Normal

Elsewhere in PokéAPI

1 more holder without a page here

Alternate forms and other species that PokéAPI lists with Swarm.

Orbeetle Gmax