Is it possible for a magical knight to increase his amount of mana in Black Clover?
It isn't possible for a magic knight to increase the amount of mana they have. Though a few very skilled magic knights can use a skill called mana zone which lets them use more mana than they have by taking mana from the environment. Their mana doesn't increase as they grow but the number of spells they have and the complexity and power of the spells does increase. They can also control their mana better which leads to greater power as shown by Yuno and his Spirit Dive.