Faraday's law sets the output
The overall reaction is 2 CO₂ becomes 2 CO + O₂, and each O₂ needs 4 electrons. Moles of O₂ per second = current / (4 x 96,485 coulombs per mole). Multiply by 32 grams per mole and 3,600 seconds for grams per hour, and by the number of cells, since each cell carries the same current in series.
This assumes every electron goes to making oxygen. NASA's demonstrator on the Perseverance rover made about 6 grams of oxygen per hour in its first run, and was designed for up to about 10 to 12 grams per hour.
Try it: set 2 amps and 10 cells for about 6 g per hour, then max both controls. Output reaches about 24 g per hour, yet one person's daily oxygen still takes about 35 hours.
Why oxygen from Mars matters
Rocket propellant is mostly oxidizer. Making oxygen on Mars instead of shipping it from Earth could cut the mass a return mission must launch, and the same oxygen could support crews and greenhouses.