Slots = max number of players
DDOS = Distributed Denial Of Service - a good thing to be resistant to.
So if you are renting a server, the more players you want to accommodate, the more it will cost.
You can host your own server http://hurtworld.gamepedia.com/Hosting_A_Server
Of course, that would need a decent internet connection, and some tweaks to firewall and router settings to allow the traffic.