I'm installing a vertical antenna in back yard. About 20 feet in height and using 1 counterpoise wire 16 feet for 20 meters. Does installing two or more counterpoise wires help?
A single counterpose wire makes the antenna pattern directional, usually in the direction of the counterpose. Two or more counterposes in a symmetrical arrangement makes the antenna pattern more horizontally omnidirectional.
The asymmetry of a single counterpose also makes it more difficult to arrange the feed line so that the antenna's radiation pattern doesn't couple to the feedline and generate common mode currents (potential RF in the shack, plus a further distortion of pattern).
The above are rules of thumb. Best analysis would be to put your particular antenna geometry into an antenna modeling program (NEC2, MININEC, et.al.)