Sure you can.
Open the file C:\Program Files\Steam\SteamApps\common\sid meier's civilization v\assets\DLC\Expansion2\Gameplay\XML\GameInfo\Civ5Worlds.xml
and in line 121 change <DefaultPlayers>8</DefaultPlayers> to <DefaultPlayers>12</DefaultPlayers>.
Start Civ and you can play MP with standard world and 12 players. Of course you can still easily delete/close players if you want to play with less than 12. And if you want to go back to the original settings, just change the 12 into 8 again. Easy as that.