They occasionally make new ones and judging by a website I found, there are next to no gaps in the numbering in the first 400-500 services. Which is strange because I remember when they split the 253 into the 253 and the 254 and it was probably only about 20 years ago, if that. Meaning they either deleted a 254 somewhere or they were just lucky.
The 14A became the 91 in the mid-80s.
And then there are all those buses that start with letters, seemingly random too: A few W buses up in Crouch End and another dahn sarf. The C11 used to trundle between Highgate and Camden and maybe still does...