I'm running White Plume Mountain in the 23rd. I have a potential of 6 players and have to come up with 6 x 8th level pre generated characters for them.

So far I have a rune knight bugbear fighter and an aasimar paladin. But I'm having a mind blank as what to make for the other 4 players. So thought I would ask you guys.

Might be nice to have a cleric. You can have the paladin and the cleric as tanks, while the two of them sharing healing duties will keep some other spells slots open for various smites or guiding bolt, etc.

Make a ranger -- the class often gets short shrift, but instances like this are a good way to show off how useful it can be, too. Monster Hunter or Gloomstalker, with elf or one of the subraces (I like Sea Elf, but if you want the wisdom bumps there are a few others that are good).

Are you planning on assigning characters yourself/randomly, or having each player pick who they want to play from the list?   If the third option, you probably want to come up with ten characters, instead of six, so no one is in a “I have to pick last and get whatever everyone else did not want to play” situation.  This also gives backups in case anyone dies.

If making characters for a one-shot, you do not need to overthink it - since your players are not making the characters themselves, making characters that fill basic tropes make it easier for your character to step into that character for roleplaying.

Things like Genasi Wild Magic Sorcerer; Firbolg Shepard Druid; Triton Swashbuckler Rogue, etc. are all perfectly serviceable characters that should be easy enough to quickly throw together.  If you are assigning the characters yourself or only making 6, I would say 2-3 melee characters, 1-2 magic users, 1 healer, one archer ranger or rogue.  If you are making characters for your party and go with “make ten, choose six” then just make 4 melee characters, 4 characters with range (magic or ranged attacks), and 2 healing characters and let the party decide its own makeup.

Is this in-person or online? If in-person, I would take a look at the minis I have and make characters based on them. If online, I'd look up a bunch of art and base the characters on the coolest images I found. And I agree that making more than you need is nice to give people options.