The more the Fed has bonds in their balance sheet, the more fragile they are in an inflationary crisis. If inflation goes up and interest rates go up, the fed's bonds go down in value. If the fed balance sheet becomes too much in the red, they need to print to make up the difference, which means more inflation.
"As of this year, the US Federal Reserve (blue line) now owns more Treasuries than all foreign central banks combined [..]"
So, if your debt is to the Federal Reserve, how much debt can you accumulate before it's too much? Does the question even makes sense?