I quote the Wikipedia page definition for Turnover number:
In other chemical fields, such as organometallic catalysis, turnover number (abbreviated TON) has a different meaning: the number of moles of substrate that a mole of catalyst can convert before becoming inactivated. An ideal catalyst would have an infinite turnover number in this sense, because it wouldn't ever be consumed, but in actual practice one often sees turnover numbers which go from 100 up to 40 million for catalase.
I'm trying to find reasons for why catalysts don't function infinitely in practice. My only thoughts are that maybe they eventually degrade from slowly reacting with other species they are exposed to, but then wouldn't this depend on the environment it is in rather than how much substrate it catalyses? I also assume that all product is taken away as it is produced.