How much more can the US Federal Reserve spend on COVID-19? On Mar. 26 2020, the Federal Reserve’s Balance Sheet Tops $5 Trillion for First Time. Is the Fed's spending capped at some maximum? 

I studied in Canada and am ignorant of U.S. I'm asking about just Fed, not US govt. I quote MarketWatch


  In an NBC News interview aired Thursday [Mar. 26 2020] on “The Today Show,” Fed Chairman Jerome Powell said there was essentially no limit to the Fed’s emergency lending ability.
  
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  “We can continue to make loans, and the point of all that is to support the flow of credit in the economy to households and businesses,” he said.
  
  The Fed is only limited by how much backstop it gets from the Treasury Department.
There’s no real dollar cap on what the Federal government can do, rather the limit is on what is available to buy, and how much it is willing to spend. (There are various institutional limits on spending, but those limits can be raised. This is what allows the size of government to grow in line with increasing nominal GDP.) 

Examples


It can only buy as much medical equipment as is available for sale. (As noted in a comment, it can use emergency powers to increase the supply, but purchases are still limited by what is in inventory and what can be produced in a given period of time.)
If it lends to a corporation (a big part of announced packages), there are limits to how much they want to lend to them, based on the size of the firm, etc.


One can point to limits that are created by the structure of the bond market. This limit is fuzzy, and there is no well-defined “cap.” One can point to the experience of World War II government finance as an example where the government modified how borrowing operations worked to eliminate restrictions created by the bond market.