According to Wikipedia:
“The Oort cloud may have billions of objects with absolute magnitude
brighter than 11 (corresponding to approximately 20-kilometre diameter)”
Most Oort cloud objects stay in the Oort cloud, and we never see them.
Comets don't last long. A comet is made of a mixture of ices and dust. A comet that enters the inner solar system has one of a few outcomes:
In all cases, the comet is lost, destroyed or deactivated.
As comets have a short lifespan, they are rare.