problematic douts

"Using xdata banking you may extend the address space..." (my emphasis)

For any processor, the amount of memory addressable is governed by the width of the databus (which is usually closely related to the processor's Word size).
This information can be found in the Datasheet for the specific device.
Note that not all 8051 derivatives today provide an external memory interface - so they're further limited to what's on the chip.

You can, of course, expand the addressing range of any processor with suitable hardware and software; eg, Keil's XDATA Banking for the 8051, or the disk-based Virtual Memory found on big systems.

So the question "how much maximum external memory(ram) i can access?" is really just another "how long is a piece of string?"

"Using xdata banking you may extend the address space..." (my emphasis)

For any processor, the amount of memory addressable is governed by the width of the databus (which is usually closely related to the processor's Word size).
This information can be found in the Datasheet for the specific device.
Note that not all 8051 derivatives today provide an external memory interface - so they're further limited to what's on the chip.

You can, of course, expand the addressing range of any processor with suitable hardware and software; eg, Keil's XDATA Banking for the 8051, or the disk-based Virtual Memory found on big systems.

So the question "how much maximum external memory(ram) i can access?" is really just another "how long is a piece of string?"