From OAG:

US domestic airline capacity continues to see growth this month and is now just 6% below January 2019.
What are the busiest domestic routes in the US this month?
- New York to Chicago (LGA-ORD) is the busiest with a total of 262,256 scheduled seats (a 21% increase from last month).
- In second place is New York JFK to Los Angeles (JFK-LAX) with 261, 813 scheduled seats.
- Atlanta to Orlando (ATL-MCO) has 251,029 seats making it the third busiest route.
Most popular states?
- Five of the busiest airline routes start or end in New York and five start or end in Florida.
Busiest International Route?
- New York JFK to London Heathrow (JFK-LHR) is the fifth busiest global international route this month with 213,199 seats.
See the full charts and stats here