Probably zero"? You don't sound exactly certain. Perhaps you'd like to look it up and then get back to us.
Well let's see, slavery ended about 150 years ago. So if you were around 80-100 , there's a good chance you may've known a former slave. But considering everything, it's probably extremely rare.
And I didn't make it to be a fact or a statement of fact. It was just personal musing. But it's more than likely than extremely few if any still living blacks knew any slaves.