Upto 32KB of external memory from 0x4000 - 0xBFFF is available via one chip select line (nXRAMSEL) with RAM Merge enabled (BIOS default). Additionally, another 8KB region from 0xC100 - 0xDFFF is available via a second chip select line (nXROMSEL) giving 40KB of total available external memory. Together with the internal 15KB, this gives a total of either ~48KB (1 chip select) or ~56KB (2 chip selects) of available memory for either code or data.
Please note that the memory map and pin names (nXRAMSEL/nXROMSEL) define specific memory regions for RAM vs. ROM. This allows the BIOS to look in the upper external memory space at 0xC100 for SCAN vectors (enabling code to be loaded/executed from ROM). If no SCAN vectors are required in the design (external memory is used exclusively for data), then all external memory regions can be used for RAM. Similarly, the external memory can be used exclusively for code space (ROM).
If more external memory is required, EZ-Host has enough address lines to support upto 512KB, however this will require complex code banking/paging schemes via the Extended Page Registers.