The highest Q3 shipments for the Wii was 11.31m in 2009. So even if the Switch matched the Wii at it's peak... they'd still need to ship 6.8m in Q2+Q4 combined. It seems so unlikely.
Q1 - 1.88m
Q2 - 3m
Q3 - 11.3m
Q4 - 3.8m
That's how they'd reach 20m, but I don't see them matching the Wii's peak holiday and even if they did I don't see them shipping that many in Q2/Q4. If Nintendo stick to the 20m forecast after they announce Q2 shipments I'd be very surprised.