  We study the scaling of decoherence of decoupled electron spin qubits due to
hyperfine interaction. For a superposed state consisting of product states from
a single Zeeman manifold, both $T_2^*(n)$ and $T_2(n)$ are scale-free with
respect to $n$ and the number of basis states, $m$. For a superposed state made
up of states from different Zeeman manifolds, both $T_2(n)$ and $T_2^*(n)$ are
roughly inversely proportional to $\sqrt{n}$. Our results can be extended to
other decoherence mechanisms, including in the presence of dynamical
decoupling, which allow meaningful discussions on the scalability of spin-based
coherent solid state quantum technology.
