I was on the Summit for the same cruise as the OP. A crew member told me our sailing had about 1,900 passengers; the Summit's capacity is 2,200. He said some cabins are still being left open to serve as isolation rooms if needed. The ship felt full, with elevators crowded and dining tables very close. (I have never had a problem getting a table for 2, but with only a few inches between tables, you can easily get to know your neighbors.) I also heard from crew members that they were not allowed to get off the ship in ports because of an increase in crew covid cases. I didn't hear anything about passenger cases.
A couple of crew members I spoke to said staff shortages varied by department. The only service drop-off I noticed was that it was sometimes hard to find a bar server for tableside beverage service in Oceanview and Cafe al Bacio. Otherwise, I thought the crew worked very hard and provided great service as usual. I had a wonderful cruise despite occasionally having to go to the bar to get my own drink.
As for supplies, sometimes the specific bar we were at didn't have the non-alcoholic beverages we wanted, but the bartender usually tried to track down what we wanted, or we would just go to the next bar. I didn't notice any other supply shortages --- and I actually think what we encountered was more about distribution of products within the ship than it was about an overall shortage.