i have enough materials but the game only allows me to exchange for ramen, i wanted to get keys.

http://image.prntscr.com/image/2779bebe8ffa4657861dfe69cfa62e90.png

i know it says the exchanges are limited, but it doesnt say the limit for each exchange anywhere how are we suppose to know?
and why would i want ramen when all my 70 ninjas are 99.99% and i have over 1k ot them just laying around from other rewards i naturally get?
what to do with all the extra item?