In a nuclear reaction, the quantity that remains the same before and after the reaction is the total number of protons.
In any nuclear reaction, the conservation of nucleons (protons and neutrons) is maintained, meaning that the total number of protons and neutrons in the reactants must equal the total number in the products. This is called the conservation of baryon number.
While the types of atoms and the energy can change during the reaction, the total number of protons (which determines the atomic number and thus identifies the elements involved) is conserved. Additionally, while energy is conserved overall, the distribution of energy may vary between the reactants and products of the nuclear reaction.
Alpha particles may be produced or consumed in some nuclear reactions, but this is not a universal principle, unlike the conservation of total protons.