How much RAM do the different models of iPad have?
A very nice visual layout is kept current at: iOS Support Matrix

The top row has RAM size below the horizontal line showing the late 2016 and early 2017 models have 2, 3, or 4 GB of RAM across the board. 

From Wikipedia:


1st generation: 256 MB
iPad 2 and iPad Mini: 512 MB
3rd generation, 4th generation, iPad Air, iPad Mini 2 and iPad Mini 3: 1 GB
iPad Air 2 and iPad Mini 4: 2 GB
iPad Pro 10.5 and 12.9-inch: 4 GB 
iPad Pro 9.7-inch: 2 GB