I assume the amount of oxygen MOXIE can produce is limited so how much can it produce before the solid oxide electrolyzer cell stops working?
The answer is up to around 10g/hour. The experiment will be run multiple times throughout the lifetime of the rover and each run will be for about an hour. A full scale version for use on a human mission would need to be many times larger. NASA suggests two orders of magnitude larger, but if Starship is used probably three orders of magnitude bigger.
Alternatively MOXIE might just be used in conjunction with electrolysis in order to tailor the net oxygen production. This is because Raptors burn fuel rich and because oxygen will also be needed by the crew and wasted by leakage during airlock cycling etc., so MOXIE could be used just on a small scale to balance the oxygen demand to prevent dumping excess hydrogen from electrolysis and provide an immediate source of additional breathing oxygen on Mars from day 1.
Edit: the ultimate limiting factor on MOXIE is an open question. There are a number of constraints that might cause problems such as electrode degradation, dust blocking the HEPA filters, damage caused by repeated temperature cycling (MOXIE runs at 800 degrees C) as well as the potential to over heat and the need to use the 300 W consumed by MOXIE for other things. Perseverance itself only uses 110W so MOXIE will drain the battery if used for an extended period. https://www.hou.usra.edu/meetings/ipm2016/pdf/4130.pdf