An orbit has one point for each coset of the stabilizer.Suggest a slogan
Let a group \(G\) act on a set \(X\), and let \(x\in X\) have orbit \(Gx\) and stabilizer \(G_x\). Then \[ \size{Gx} = [G: G_x], \] and if \(G\) is finite, then \(\size{Gx} = \size{G} / \size{G_x}\).
The map \(G/G_x\to Gx\), \(gG_x\mapsto gx\), is a well-defined bijection of sets, which gives the equality \(\size{Gx} = [G:G_x]\).