  We report electron spin resonance measurements of donors in silicon at
millikelvin temperatures using a superconducting $LC$ planar micro-resonator
and a Josephson Parametric Amplifier. The resonator includes a nanowire
inductor, defining a femtoliter detection volume. Due to strain in the
substrate, the donor resonance lines are heavily broadened. Single-spin to
photon coupling strengths up to $\sim 3~\text{kHz}$ are observed. The single
shot sensitivity is $120 \pm 24~$spins/Hahn echo, corresponding to $\approx 12
\pm 3$~spins$/\sqrt{\text{Hz}}$ for repeated acquisition.
