Apple's M1 iPad Pro is available with either 8GB RAM or 16GB RAM, the most memory that's ever been made available in an iPad model.
iPad Pro models with 128GB, 256GB, or 512GB of storage will come with 8GB RAM, while iPad Pro models with 1TB or 2TB of storage will feature 16GB RAM.
It does not appear that there will be a way to customize lower storage iPad Pro models with the upgraded RAM, so those who want the 16GB RAM option will need to opt for higher storage tiers. Both the 11 and 12.9-inch iPad Pro models are available with up to 2TB storage.
The iPad Pro will be available for order on April 30, but orders will not ship out until late May.
Source: Macrumors