Is it possible for a magical knight to increase his amount of mana in Black Clover? In Black Clover, some magic knights run out of mana quickly after a few spells and others can use a lot of spells because "they have a lot of mana". How does this people to have a lot of mana? Can they train to increase it? Is the mana you are born with the only mana you'll ever have? 

To summarize the question, 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.