To host more than 48 slots on your gameserver a Patreon membership is required.

 
Patreon memberships:

FiveM Element Club Argentum 💿 for more than 64 slots 

FiveM Element Club Aurum 📀 for more than 128 slots

FiveM Element Club Platinum 🌟 for more than 2048 slots

You can book a membership here: https://patreon.com/fivem

After booking your membership you have to go to https://www.keymaster.fivem.net. Here you will need to create an account with the same email address you previously used to purchase your Patreon membership.

After that you can add your key in your server.cfg of your fiveM gameserver. After you have added your key, you still need to activate "OneSync".

Activate OneSync:

"onesync_enabled true"