QUESTION:
How many potato plants does it take to supply one person with enough oxygen?

ANSWER is from Cheryl Mackowiak on July 30, 1997:
http://atlas.ksc.nasa.gov/celss/STAFF/staff.htm

It has been reported that the average human needs 0.83 kg of oxygen each
day.

Oxygen regeneration from plants is dependent on photosynthetic rate and this
is dependent on various environmental factors.  In our system (20/16 C
light/dark, at a 12/12 hr. photoperiod and CO2 at 1200 ppm), it would take
171 potato plants or 26 m2 growing area.  Wheat can be grown at a much
greater density and so it would only take 19 m2 growing area (11,937
plants).

When looking at plant budgets, it is more useful to present information in
terms of growing area or dry mass (g) than to use the number of plants. 
Plants vary in size relative to individuals, cultivars or species.  I
suggest you use growing area in the future.

Hope this helps,
Cheryl