If you need external ram and need a lot of i/o ports then:

a) Buy a derivate with more ports and interface the ram normally

b) Select a serial ram/eeprom

c) Conect an standard wide bus ram and change some i/o as memory maped.

d) Standard ram and serial i/o expander


The solution, depends of your data flux. For example if you need a very high transfer of datas to the ram and some inputs are not very used then you can select the d option.