Do we know how much Switch sold in the US by the beginning of December? We can use that information to calculate the December sales in the US. The charts unfortunately only show regional data in the US up to November 18.
Based on the yearly USA numbers for hardware, which I believe goes up to the most recent hardware update for 02 Dec, the Switch had 3,738,819 units sold. So, assuming the 4.8 million is sold through, over 1.1 million Switches were sold in Dec.
Feel free to correct me if I'm wrong. The site can be a bit confusing as far as obtaining exact numbers for specific dates.
Edit: Looks like someone used NPD numbers, so I'd take their word for it over VGchartz