quantum field theory - difference between classical vacuum solutions and instantons

What does the classical vacuum of the $SU(2)$ Yang-Mills action correspond to? Does it correspond to $F_{\mu\nu}=0$ everywhere or just at the spatial infinity? In Srednicki’s book, he has shown that, there are infinite number of classical field configurations of zero energy, each charecterized by a winding number $n$. I believe, instantons doesn’t require $F_{\mu\nu}^a=0$ everywhere but only at the boundary. Am I right?

In Ryder's book, he talks about instantons but does he really shows that how SU(2) Yang-Mills theory has an infinite number of degenerate vacua?