I ran a battle royale style one shot for a big group recently, based on the Battle Royale module from DMGuild.

Because it was such a large group (11 players) I made a bunch of compromises:

• initial 2.5hrs was team based, and sold to the players (in character) as if they could achieve victory together
• during this period, each area/activity they wanted to achieve was run as a skill challenge
• 6 success before 4 failures to succeed on the challenge, DC changed depending on the activity
• expending a short/long rest resource or spell slot could get advantage on the roll.
• I kept it to 2 areas where they could acquire loot, 2 random travel events and a final longer (8/4) section of the final circle
• in the PVP section, the in-game game master announces they must fight on to the death
• again with this many players I needed to try and keep the game moving, I used NPCs and lair events to keep from players from hiding in the corner too long
• as rounds went on I added another d6 to the lair action damage at the top of the round. Eventually I started upping the DC on the dex checks just to keep things moving as well.

As the other comment said, it was really important to be upfront with the players, out of game, about the challenges of PVP. It was unavoidable for players to be sitting around waiting for the combat to finish with little to do.

I also found delegating some tasks made it easier for me to keep a handle on such a large group:

• timekeeper, trying to keep us to the limits I set at the beginning
• loot giver, I’d announce how many/type of chest they found which corresponded to how many times they could roll on the loot table, and on which one. One IRL player did this for me and distributed as best he could