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.
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