How to change your player slots: (Pterodactyl Panel)
1) Login to your control panel and select the server you want to edit the player-slots.
2) Locate File Management and locate server.properties
3) Click on the server.properties file and look for max-players
4) Now change the number after max-players= to any amount that your prefer for your server slots.
5) Restart your server
How to change your player slots: (Multicraft Panel)
To change your slots in Multicraft, press servers, select the server then change the slots in "Player Slots" http://prntscr.com/i6xwnx