It is how many of those towers you have available to build. For example, if it says you have 5x "Sting" towers, the number will go down after each one you build, until you have 0 and the tower becomes "not available".
Your content is saved locally, and making backups of those files is up to you (the game tells you that whenever you create locally saved content.) See http://www.kongregate.com/forums/9794/topics/521014 for more info.
 "Idle Wizard"){kind=icon}
 "Monster Tiles TD"){kind=icon}
 "Cividlization 2"){kind=icon}
 "Raze 2"){kind=icon}
 "Press The Button"){kind=icon}
 "Mutilate-a-Doll 2"){kind=icon}
 "Logistics Inc"){kind=icon}
It is how many of those towers you have available to build. For example, if it says you have 5x "Sting" towers, the number will go down after each one you build, until you have 0 and the tower becomes "not available".