The magnitude of vector $ \vec{v} $ is $ \| \vec{v} \| = 12 $ .
To find magnitude of a vector $ v=(a,b,c) $ we use formula $ ||v|| = \sqrt{a^2+b^2+c^2} $
In this example $ a = 12 $ , $ b = 0 $ and $ c = 0 $ so:
$$ ||v|| = \sqrt{ 12^2 + 0^2 + 0^2} = \sqrt{ 144 + 0 + 0} = \sqrt{ 144 } = 12$$