Classical and neoclassical music have one major difference: the neoclassical music has a bigger orchestra. The classical music is often done on just a single piece: a piano (think Beethoven) or a violin of some kind. Or, in the case of classical music that requires more than one part, there are only three or four different parts. Neoclassical music has parts for all different kinds of players, and often requires more than one of each instrument so that the balance is heard.
A neoclassical piece will often have parts for over 100 instruments. This may sound like a lot, but it’s the basis of the modern orchestra. Modern orchestras can have anywhere between one hundred and two hundred instruments and players.