Horizontal Asymptote
Horizontal asymptotes occur when a function approaches to a certain value as x goes to infinity. So, there can be 2 different horizontal asymptotes at max. One for $\infty$ and one for $-\infty$. In order to calculate a horizontal asymptote for $f(x)$, you need to calculate $\lim_{x\to \infty}f(x)$ and $\lim_{x\to -\infty}f(x)$.