Current biggest on amazon : 1.5TB (1.5To)

From tech radar :
https://www.techradar.com/pro/largest-microsd-cards#:~:text=What's%20the%20biggest%20microSD%20card,to%20depend%20on%20market%20conditions.

What's the biggest microSD card on the market today?

Micron's 1.5TB microSD card is currently the largest one available for purchase.  Kioxia unveiled a 2TB Exceria Plus prototype back in September 2022 but while a 2023 launch has been suggested, it is likely to depend on market conditions. 

What's next for microSD cards?

Aside from the exciting news about Kioxia's upcoming 2TB SD card, it's likely we'll see huge improvements in both microSD card capacity and speed in the near future. The most recent 'U3' (UHS Class 3) has an average write speed of 30 MB/s, capable even of writing 4K video in real time.

V60 and V90 video class SD cards currently offer the fastest speeds available making them suitable for 8K resolution videos in drones, high speed cameras and similar devices. It's likely we'll see even faster microSD cards for more reasonable prices in the future. 

In terms of capacity, the SD Association has stated that thanks to the new SD Express and SDUC (SD Ultra Capacity) standards, the potential maximum memory capacity has increased. 

Previously the old SDXC standard limited SD cards to a maximum of 2TB but thanks to SDUC, this can potentially be increased up to 128TB. This will require new drivers and formatting to work correctly in all devices however, so it may be a while before we see these on the market.