Zagato officially bodied four cars only and that was back in the late Fifties on 250 TdF chassis.
Zagato did a few more in the Seventies (a 330 or 365 GTC, or GTS), plus did a Testarossa in the Eighties and a limited series (18 or so) of 348's in the Ninties. Customer-ordered only, after the stock car was produced. The TR was an exercise and only used for auto shows.
I may have forgotten one or two more from Zagato.
Other Coachbuilders in no particular order:
Abarth (one 1953 166MM)
Bertone
Vignale
Stabilimenti Farina (before it became Pinnin Farina)
Ghia
Drogo
Fantuzzi
of course Scaglietti (before they became the "house" coachbuilder for PF)
Boano (later became Ellena)
All I can think of for now. There was a Swiss company called Felber that made one or two goofy-looking cars in the Seventies, but I wouldn't consider them as serious production.