  We report electron spin resonance spectroscopy measurements performed at
millikelvin temperatures in a custom-built spectrometer comprising a
superconducting micro-resonator at $7$ GHz and a Josephson parametric
amplifier. Owing to the small ${\sim}10^{-12}\lambda^3$ magnetic resonator mode
volume and to the low noise of the parametric amplifier, the spectrometer
sensitivity reaches $260\pm40$ spins$/$echo and $65\pm10$
$\mathrm{spins}/\sqrt{\text{Hz}}$, respectively.
