server.cfg in the root folder of your FiveM server.On a FiveM server from HostValues you can edit the server.cfg through the game panel under Files.
The server.cfg is the main configuration file for your FiveM server. It controls the server name, max player count, resources and convars. FiveM reads this file every time the server starts.
For most servers, yes. OneSync improves synchronization with more than 32 players and is required by popular frameworks like ESX and QBCore. Without OneSync you will hit limits at higher player counts.
You generate a key at keymaster.fivem.net. You need a Cfx.re account. Register the IP address of your server and copy the key into sv_licenseKey.