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 $260pm40$ spins$/$echo and $65pm10$ $mathrm{spins}/sqrt{text{Hz}}$, respectively.
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