Cracking it is a bit tough .......no man I am kidding ....Just apply your senses

See its a bit tricky The numbers given are in alphanumeric sequence with each number being the largest digit possible using three letters ,four letters........and so on

So $10 ----> three$ $9 ------> four$ $60 ------->five$ $90--------> six$ $70--------->seven$ $66--------->eight$ so for nine digits the largest number is --> $\boxed{96}$