I posted this in another thread, and I added a few things for clarification
If we expect 51M shipped by March 31st (From Nintendo Financials), then at a production rate of 2.4m/month, then we can expect ~43.8m shipped through December 31st. Now, we could assume that they will air ship 2 extra million from january's supply. So assuming you have 0.5m in stock/transmit for the Dec 31 deadline, that would leave you with a max of 45.3m units sold to customers by Dec 31.
Of course, this is with a few assumptions, mostly that they will have 2 extra million they can divert from January to December, but I honestly expect 44-45m by Dec 31.
I did some estimates from last year, and it does look like they can shift that much production forward.