That is a very difficult question to answer.
We don’t really know.
The only way we could find out is by trying to infect a person in the lab and that is not allowed of course, because we would be making somebody sick on purpose.
If we ever find a very effective cure for COVID-19, so that we could absolutely guarantee the safety of the test subject, such an experiment could be done.
I suspect the number may also vary from person to person. For example, somebody with a very strong immune system may not get very sick from the same amount of particles that could make somebody else seriously ill.
