Nintendo has confirmed that, while not available just yet, the Nintendo Switch will support 2 terabyte micro SDXC cards.
In a statement to Game Informer, Nintendo stated:
Nintendo Switch is compatible with the SDXC standard, which supports up to 2TB. (Note that 2TB cards are not yet on the market, but the system will support them when they are.)
While 2TB Micro SDXC cards sound great, they are still a good ways off from hitting the market.
The biggest size available is 256 gigabytes and even that costs about 200 dollars on the low end. However, it is good to know that the console will support the most storage possible as it comes available.