The way Space Engineers is currently setup, it has two config files which contradict each other these are:
1. Our Main Config File (read for each new map generated)
2. The individual map config file (this overwrites our main file)
Step 1:
Log into your game panel and click 'stop' on your service.
Step 2:
Select File Manager from the icons

Step 3:
Once in File Manager navigate to /settings/saves/Your Loaded Worlds Name
Step 4:
Find the file called "Sandbox.sbc" and edit it using the icon
We suggest setting this as a favourite by clicking the star icon
Step 5:
Navigate through the file until you find the line:
<MaxPlayers></MaxPlayers>
Step 6:
Set this back to the slot count you ordered
*Don't set this higher than you ordered - See Note
Note: Don't set your server to a higher slot count then purchased as our systems are set to stop and automatically suspend services that have a higher slot count than they should.