The flu season starts in October, and
estimated deaths as of Mar 28 are 24000-63000. So, 4000 to 10,500 per month over six months. October and November are at the low end of the curve, so even putting the whole count into the December-March four months, the range is 6,000-15,750 per month.
Note that I made that four-month example because the first two months of flu season have relatively few deaths. the 12-13000 count for COVID-19 is for its first month.