Do some people who become infected with coronavirus COVID-19 and appear to recover, remain carriers who can still infect others? What is the percentage?

This is very true as well. It is, in fact, a third or more of the victims who remain in a state of viremia and will shed the virus in their secretions and excretions depending on the buildup, distribution, and other characteristics of the individual. It will vary and may wax and wane as well in the level of virus present in the shedding.