how to limit the number of characters?

Hi all
first of all thanks to all the hashcat community for the work and support.

i need to crack a password that i know is 18 characters long , it has no numbers, it can be upper case and lower case, and i know that its a phrase taken from a book.

so how can i limit the amount of repeated letters in the password
hoe the mask would look like?

if the password is 16 characters long i dont think a letter can appear more
than 5 or 6 times

thank you all.