- Rod Serling wrote so many "Twilight Zone" episodes that I can't even be prevailed upon to count them.
- Carl Reiner wrote the vast majority of "Dick Van Dyke Show" episodes for the first two seasons.
- Jay Sommers co-wrote nearly every single episode of "Green Acres," usually with Dick Chevillat.
- Paul Henning co-wrote every episode of "The Beverly Hillbillies" and "The Bob Cummings Show."
- Joe Connelly and Bob Mosher wrote nearly every episode of "Leave it to Beaver" for the first four seasons or so.
Oddly enough, as the number of episodes ordered per season started to decline (from 39 per year to the current 22-24), so did the number of scripts written per year by individual writers; the creation of the position of showrunner -- fusing into one the functions previously performed by several different people -- probably made it harder for a creator to devote his or herself totally to the writing. "Soap," where Susan Harris co-wrote every episode, was one of the few '70s shows that reverted to the system of having the creator supervise the writing and leaving the administrative duties to non-writing producers (Paul Witt and Tony Thomas). And of course, there's David E. Kelley, but let's not go there.
As to whether it's good for one or two writers to write everything, I actually have my doubts that this was the most effective system. It's effective on shows with short seasons, as in England, where you produce 6-13 episodes and thereby allow one writer or team of writers to write everything. But with 30-39 episode seasons, even the good writers sometimes seemed to spread themselves too thin. Rod Serling is the classic example: his contributions to "The Twilight Zone" were astoundingly uneven, with a great script one week followed by a terrible script the next. Outside contributors like Richard Matheson and Charles Beaumont arguably did better work for the show than Serling did, and there's a strong argument to be made that the show would have been stronger if Serling had written fewer scripts himself and commissioned more scripts from other like-minded writers. And "The Dick Van Dyke Show" improved quite a bit in its third season when Reiner finally took some other writers on staff, specifically two talented young teams from New York: Bill Persky and Sam Denoff, and Garry Marshall and Jerry Belson.
On the other hand, some shows are so quirky that it's doubtful that anyone except the creator can write for them at all. "Soap" was like that, and it collapsed in its fourth season when Harris turned over some of the work to new writers; the show depended so much on Harris's writing idiocyncracies that it seemed pointless when anybody else was writing for it.