[sciolistse]

It's true the NES has 2 KiB built-in RAM, but a cartridge can provide its own RAM. Some of the later cartridges (MMC5 for example) supported up to 128 KiB of extended RAM. There's not really any limit to how far you could push that with a custom mapper.

[mikepurvis]

The extreme end of that is just putting the entire system in the cartridge and using the NES itself for IO only, for example:

https://magpi.raspberrypi.com/articles/reverse-emulated-nes