How to Increase the Slots on a Minecraft Java Server
Open server.properties
Connect to your server via SFTP and open the fileserver.properties.Find the entry
Look for the following entry:max-players=20max-players=20Change the value
Change the value to the desired number of players.Restart the server
Save the changes and restart the server.
Warning
Make sure that your server has enough resources (such as RAM and CPU) to support the increased number of players.