In most cases where the value of X isn’t defined in the text of a card, its controller chooses the value of X. However, the X in King Darien’s name is always 10.

XLVIII is forty-eight.

If King Darien XLVIII leaves the battlefield, other creatures you control immediately lose the +1/+1 bonus. Notably, this means that if you activate its last ability after a creature has already been dealt damage, it might die due to having lethal damage before the ability resolves and gives it indestructible.