see Bank Switching
Menu Driven Megacart
see AtariAge: Menu-Driven Multicart for Atari 2600
In Stella, and as described here, the hotspots are from $800 to $BFF instead of $800 to $FFF.
The hotspots in this scheme are read/write at addresses $800 to $BFF, where the lower byte determines the actual 4K bank switch to. In the current implementation, only 128 banks are supported, so selecting bank 128+ results in further bankswitching being locked. A reset line is used to reset to bank 0 and re-enable bankswitching.
Therefore, there are 128 banks / 512K possible in total.