The one in between seven and nine. You know, two cubed. Four plus four. The one you can't say out loud or else you'll get ate. What do you mean, the ichor god Bel-Shamharoth can't tell the difference between two homonyms like the past tense of the verb "to eat", and the number eight... OK, so they sound identical but have different meanings, that's what a homonym means. Fine, but is this really the time for a grammar lesson?
There are 7a Great Spells in the Octavo; when uttered in The Light Fantastic, they facilitated the hatching of 7a baby world-turtles.
7a was Rincewind's room number in his Hall of Residence. He wasn't surprised.
The Opera Ghost demanded sole use of Box 7a on all opening nights.
The book Going Postal was the first Discworld book to be divided into conventional chapters. The chapter in between Chapter Seven and Chapter Nine is numbered Chapter 7a.
During the wizards' second sojourn in Roundworld, Ridcully demonstrated that magic was non-functional there by repeatedly uttering the number between seven and nine, with no adverse side effects. Well, none aside from having to drag Rincewind back out from under the table.
Jeremy Extispicy is listed in The Compleat Ankh-Morpork as a Clairvoyant and Sausage-Maker. His possibly unique business is based at 7a Marrowbone High Street. So even street numbers avoid using the one which is (twenty-four divided by three). The Post Office must adjust accordingly, then.
Whichever way you look at it, this is not a lucky number on the Disc. If triskaidekaphobia is the irrational fear of the number thirteen in any of its associations on Roundworld, there must surely be a parallel condition on the Disc, called perhaps octophobia...
It could also be noted that in China, the number four carries associations of bad luck and death, and there is the Japanese word shi ni (4 x 2) which translates to 'death'.