The net quark number of an isolated system has never been observed to change. V A difficulty with the experimental investigation of quarks is that an isolated quark has never been observed. Quarks are always confined in compound systems which extend over distances of about 1 fm. The most elementary quark systems are baryons which have net quark number three, and mesons which have net quark number zero. In particular, the proton and neutron are baryons.
Mesons are essentially a quark and an antiquark, bound transiently by the strong interaction field.
The term hadron is used generically for a quark system. The proton is the only stable baryon. All mesons are unstable.