The US Central Bank, the Fed (The US Federal Reserve) has a vast balance sheet

https://www.federalreserve.gov/releases/h41/current/h41.htm

Just over $4,500 Billion ($4.5 Trillion)

One line item of the assets that makes up the $4,500 Billion are mortgage back securities.
These are effectively the debt of mortgages that the US Fed now owns after its programme of bond purchases since the start of finance crisis.

These are worth $1,762 Billion.

Effectively these are mortgages owned by the US Fed. US homeowners are paying the debt back via their monthly payments.

One can make some very rough calculations on this $1,762 Billion figure

If the average US house price is say $190,000, and say the mortgage on this house is say $175,000.

One could make a rough guess of mortgages held by the US Fed.

Mortgage Back Securities owned by the US Fed: $1,762 Billion

$1,762 Billion = $1,762,000 million

Average US Mortgage = $175,000 = $0.175 million

This number of mortgages owned by the US Fed = $1,762,000 million / $0.175 million

Thus = 10,068,571 Mortgages.

That is over 10 Million US Mortgages (homes) are owned by The US Federal Reserve.