With larger groups I tend to stick to less mechanically complex games.
Most OSR games can be run on the fly with any number of players. I had a fixed group of 9 run through Keep on the Borderlands, with 1 or two extras jumping in for a session here or there.
My absolute favourite is Savage Worlds. It'ss great as the maths isn't tight and "balancing" an encounter is just a matter of throwing in more mooks, throw in a wild card per 2 or 3 players. It can fit to any setting, though I strongly recommend Deadlands.
My close second favourite is Call of Cthulhu, which I've run with 8 players. There's not a combat focus so sessions are unlikely to get bogged down, and even then, most combat actions are a simple contested roll. Investigations tend to resolve as people splitting into pairs and following different leads; two go archiving at the library, two visit a sanitarium patient, two head over to the local paper to see if any stories have been published or even blocked by an editor, two stake out points of interest.
Maybe there should be a piece that tells a story beyond "nyah, I'm evil!"
Any manner of wizard should really have a personal ritual site, allowing them to bask and practice at astrologically appropriate times. Sacrifices on the equinoxes to ensure a bountiful summer and a mild winter; Fires on the solstices in appreciation of said summer/winter; charge under the waxing moon in anticipation of a particularly challenging ritual; dampen troublesome magical side effects under the waning moon; clear your mind under the new moon; channel power of the full moon into your key rituals.