If you need to specify the number just use it - like English. But if you want to say I have many cars - Saya ada banyak mobil mobil. Now most people drop 2nd mobil as using word banyak indicates more than one.
South Walian ya peasant ... Matth is north Walian so it would be "Fishee-wishee(s)" ... in my opinion Now grow up
How about just adding 'es' in all such cases instead of fcking around changing the letters at the end. radii... radiuses loci... locuses