You can change the player slots on a Minecraft Spigot server by editing the server properties file.
1. Go to the "server.properties" file, located in the root folder of your Minecraft server.
2. Open the "server.properties" and look for the "max-players" setting.
3. Change the value to the desired number of player slots.
4. Save and close the file.
5. Restart your Minecraft server for the changes to take effect.
Note: The maximum number of players that can join the server is determined by the available system resources and the max-players setting in the server properties file. It is recommended to set the max-players setting based on the server resources, such as RAM and CPU.