To add more players, you will need to download the Sourcemod and Metamod in order to have plugins and extensions that allow you to host more players.
Sourcemod[www.sourcemod.net]
Metamod[www.sourcemm.net]

Open the Winrar file (or another program to extract the files) and drag the "addons" and "cfg" folders to this location:
Steam\steamapps\common\Left 4 Dead 2\left4dead2

Then in launch options you put the command -insecure to activate the plugins.

1. Right click on Left 4 Dead 2.
   
2. At the bottom of everything in Properties.
   
3. And in the part where it says "Launch Options", there you put the -insecure

Now you will need to have Admin permissions. For that we go to the following location:
Steam\steamapps\common\Left 4 Dead 2\left4dead2\addons\sourcemod\configs

And in the file where it says "admins_simple". You open it and put your Steam ID and with the "99:z" flag to have a higher value as Admin.

If you don't know what your Steam ID is, copy your link from your Steam profile and place it on this page: Steam ID[steamid.xyz]
There it will tell you your Steam ID and put it in the file. For example like this: "STEAM_0:0:********" "99:z"

Ready, you have Sourcemod and Metamod installed and you are now Admin. You can even use it on your Local Server in case you don't have a Dedicated Server.

Note: Keep in mind that having the -insecure command, you will not be able to access servers protected by VAC. Don't worry, they won't ban you, but they'll kick you automatically for having plugins installed.

Now what is expected. To host more players, follow step by step to install the plugins and extensions.

All 8 Survivors[forums.alliedmods.net]

And they have to have this mod to be able to host the 8 players in the lobby and that they can join their server.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2754956355