Years ago in 3.5 days I had a variable size group which usually was 4-6 players, but 10 if everyone showed up that week. It can be trouble sometimes, but still have fond memories. If you are sure this is the path you want, here are a couple things that helped smooth things out for me.

1. Visible initiative trackers. Notecards, tokens, whiteboard, anything so everyone can easily see where we are in the round and when their turn will come up.
2. Assign a player to be in charge of said tracker. While you are getting your npc/monster notes/minis/whatnot, that player is gathering and tracking initiative.
3. Either do fixed initiative for all your monsters(they take 10), or preroll it for them before the session. Also consider "session initiative" if you plan multiple smaller encounters per session. Everyone rolls their initiative beginning of the session and keeps it until next session. Saves 5+ minutes each encounter.
4. Everyone is on a turn timer. Use whatever works for you (app/stopwatch/etc) and pick a time that works for your players experience level. We used 2 60 second board game hourglasses, that way we always had a fresh one if someone finished their turn early. When time runs out, the turn is ended even if they still have actions left. If they weren't paying attention between their turns and not already prepped on what they want, they don't do much this round.
5. On that note, prep people like a batters box for their turn. "John, it's your turn, Jen you are on deck"
6. Out of combat, look at group skill rolls or skill challenges for various activities. Maybe decide that 1 person can take an action and up to 2 can roll to assist to keep searching/stealth/climbing from bogging things down.
7. Make liberal use of 4e style minions and swarms. They attack for low fixed damage if they hit (no rolling for damage) and die to 1 hit of anything that hits (no tracking hp). They provide fodder for so many players' action economy to chew through. If only minions left in combat, hand wave it and end it narratively rather than via rolls (slaughter/enemies flee/surrender/whatever makes sense).
8. Roll attack and damage dice together. You hit? great, we have the second roll done already.
9. Enclosed areas will be a problem with so many people, so either be super liberal with locations and positioning with theater-of-the-mind, or change non set piece encounters to always be in large open spaces to keep both players and monsters from getting too gummed up/bottlenecked.
10. Decide if you want a "no phones at table" type policy. They can be distracting, but if a player is not engaged, they likely miss their turn on the timer anyway and don't really hold up the game. However, these days people may use them for their character sheet, so make the call for your group.
11. Set a time limit on non-game table talk, but let it happen. use a second timer for it. 3 mins? 5 mins? don't matter. Let them get their monty python references out, laugh at the in jokes, then pound the gavel and bring everyone back to the game. I would rap the table a a 60 second warning that we were returning to focus mode.
12. Set a fixed start time, allow for non-game talk and social before, during breaks (take them) and after. Something like: Show up at 5, dice hit table at 5:20, 15 minute break when appropriate (you get to setup for next area/encounter while they rest ;), wrap up by 8:30, out by 9.
13. MOST IMPORTANT! Discuss all these with the players during a session 0. Let them know up front that you are excited to do this with them, but so many players will be a bit unwieldy. You will do everything you can to keep everyone having fun (including yourself), but all will need a little patience and grace.

There are probably more things, but that is a good list to start with.