Well ya see I won't be able to make a really good list but I'll try^-^
Creatures
Apocalypse hydra
Genesis hydra
Kalonian hydra
Managorger hydra
Mistcutter hydra
Primordial hydra
Vastwood hydra
Broodmaster hydra
Purphorus, God of forge
Nylea, God of the hunt
Courser of kruphix
Karametra's Acolyte
Voyaging satyr
Rofellos, Llarower emissary (banned in EDH or commander format)
Enchantment
Choke
Leyline of lifeforce
Leyline of vitality
Mana reflection
Lands
Nykthos, shrine to nyx
I know that's not twenty but if you get the land, kametra, mana reflection, Voyaging satyr, and/or to fellow Llarower emissary you have a large amount of mana to summon your hydra creatures. It's an aggro set up^-^
Creatures
Apocalypse hydra
Genesis hydra
Kalonian hydra
Managorger hydra
Mistcutter hydra
Primordial hydra
Vastwood hydra
Broodmaster hydra
Purphorus, God of forge
Nylea, God of the hunt
Courser of kruphix
Karametra's Acolyte
Voyaging satyr
Rofellos, Llarower emissary (banned in EDH or commander format)
Enchantment
Choke
Leyline of lifeforce
Leyline of vitality
Mana reflection
Lands
Nykthos, shrine to nyx
I know that's not twenty but if you get the land, kametra, mana reflection, Voyaging satyr, and/or to fellow Llarower emissary you have a large amount of mana to summon your hydra creatures. It's an aggro set up^-^