A guinea pig can poop more than 150 times a day. Not all of the waste will end up in the cage because the rodent will eat the soft stool.
The guinea pig must eat the poop to thrive and maintain a functional digestive system, and if it can't it may become impacted or malnourished. If the animal is large or is pregnant and can't reach its rectum with its mouth, some feces should be left on the bottom of cage for the animal to ingest. There are pads that can be used to litter train guinea pigs and deal with the fecal matter the rodent doesn't eat.