Specific distortions of the EPR signal of bulk anthracite are observed at low temperatures. They are accompanied by variations in the microwave oscillator frequency and are explained by the manifestation of the Rabi splitting due to the strong coupling between electron spins and the cavity, combined with the use of an automatic frequency-control (AFC) system. EPR signals are recorded at negligible saturation in the temperature range of 4–300 K with use of the AFC system to keep the oscillator frequency locked to the resonant frequency of the TM110 cylinder cavity loaded with the sample. For the sample with a mass of 3.6 mg the line distortions are observed below 50 K and increase with temperature lowering. The oscillator frequency variations are used to estimate the coupling strength as well as the number of spins in the sample. It is shown that the spin-cavity coupling strength is inversely proportional to temperature and can be used for the absolute determination of the number of spins in a sample. Our results indicate that at low temperatures even 10^16 spins of the anthracite sample, with a mass of about 0.5 mg, can distort the EPR line.