The keys are actually pens you sign your events with, and you can have as many different pens, as you want.
Discussion
It is an "n npub to n entities" relationship.
One of the entities could be a human, a group of humans, a car radio, a bot, or a cat.
You could have a pen you use for your home appliances and your door lock.
You could have a pen you use for your picture gallery and another for your tweets.
You could have a pen for zapping your taxi driver and another for paying your utility bill.
A pen for your medical records and a special one for your love letters.
