It depends what you mean by parts, e,g, Cylinder, cylinder head, tappets, rocker gear (many parts here), push rods/cam belt, cam shaft, various gears to drive cam belt or camshaft, rocker box cover, piston, piston rings, gudgeon pin, small end bearings, con rod, big end bearings, crank shaft, crank shaft bearings, engine case, oil pump (at least 4 bits in this), sump, dip stick, distributor and gears (many parts in this), leads, sparking plugs, coil, fuel pump (many parts again), carburettor or HP pump injectors etc. So you can see it's not an easy question to answer. And the answers would be different for a diesel or two stroke engine, flat, 'V's' and straights not to mention a Wankel which must have the fewest moving parts.