DCS World

Digital Combat Simulator World (DCS World) is a free-to-play digital battlefield game, focusing on a military aircraft simulation. It includes a FREE Su-25T attack aircraft and it is extensible through additional DCS modules as well as user-made add-ons and mods.
DCS World includes a vast mission area of the Caucasus region that encompasses much of Georgia - the location of the Russian invasion of Georgia in 2008. Additionally, DCS World includes:

• Powerful mission and campaign editors
• Multiplayer
• Massive inventory of air, land and sea combat units and weapons
• Advanced AI
• Fast mission generator
• Dynamic weather and seasons
• Training missions
• Mission replay system
• Pilot log book

This all allows the creation of engaging, real-world combat missions in this flashpoint region. DCS is a true "sandbox" simulation that can and will cover multiple time periods covering many types of combat and civilian units. DCS World allows both realistic game play and more relaxed game play to suit the player.

DCS modules that can plug into DCS World can include aircraft, maps, ground units, campaigns, etc. Not only can DCS World include modules developed internally by Eagle Dynamics, but it can also include those by certified third party developers. The first third party aircraft will be the UH-1H Huey.

Source: The Game Website.

About the maximum number of players: technically its 64, but as you create the server, it lets you enter the number of slots manually. You can enter 128 an it will work (tested).

However, due to a lot of bandwidth and computational power required to run such a server, it's not practically possible. Currently, even the top public servers start to experience issues past 25 players. It becomes more evident with extensive use and movement of CA groups as well.