The reaction is $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$. 1 mole of CaCO₃ requires 2 moles of HCl. Since only 1 mole of HCl is available, it is the limiting reagent and will react with 0.5 moles of CaCO₃. Remaining CaCO₃ = 1 - 0.5 = 0.5 moles. Number of molecules = $0.5 \times N_A = 3.01 \times 10^{23}$.