TV show with smallest regular cast?

Does Charlie Rose count?

Vic Perrin was the Control Voice for Outer Limits.

He was also in Star Trek (TOS) several times.
He was the voice of Balok in “The Corbomite Maneuvor”

The Metron voice in “Arena”

Nomad voice in “The Changling”

And Tharn in “Mirror, Mirror”

Well, okay, the original show. Conway was the narrator of the 1990s version.

How about Hey Vern, It’s Ernest! ?

Didn’t Varney do most of that himself?

Yeah—the only other castmember seemed to be that radio sound-effects guy (they seemed to have worked together throughout Varney’s life) who, every week, introduced the mechanics behind a “new sound effect” that invariably involved that good old second-grade standby; putting one’s hand over mouth and forcefully exhaling to imitate flatulence.

Unless you count those clips of kids saying random things (while filmed against a flat black background) that were shown between stunts.