Scott Hamilton has a new projection regarding the production gap of the B777 Classis (see here and here).
Looking at my own model I come to similar conclusions. According to my information, which might not be the newest and most accurate any more, but should not be too far off the current planning at Boeing, the complete production rollover from the current B777 Classic to the B777X should be done by early 2023, as is shown in Scotts picture.
According to my math and including the six aircraft which were booked in January I get a gap of 249 aircraft which needs to be sold to keep the production rate at 7.33 until the B777X takes over full production in 2023. I started the 7.33 rate in Q2 2019 here, when the B777X "feathers in" the production (using "Boeing tech talk" here).