Survivors slots correspond to the number of survivors you can have in your camp. It is possible to get more using gold or buying bundle (the only ways). The current maximum slots is 20. (1.5 update)
With 6 Survivors classes it would be wise to get 12 or more slots, to be able to have 2 of each or any combination of classes you desire. Deadly missions cannot be done without having 6 survivors.
| Slots | Gold |
|---|---|
| 5 | (inital) |
| 6 | 50 |
| 7 | 100 |
| 8 | 150 |
| 9 | 200 |
| 10 | 300 |
| 11 | 400 |
| 12 | 400 |
| 13 | 500 |
| 14 | 500 |
| 15 | 500 |
| 16 | 500 |
| 17 | 650 |
| 18 | 650 |
| 19 | 650 |
| 20 | 650 |
If you retire a survivor you do not lose the equipment it has on.
You also do not lose the slot, you will be able to put a new survivor in the new empty slot.
One free slot has been given in update 1.5
The slots obtained with bundle make the cost rise. It might be wise to spend gold to gain a slot before buying a package depending of the current cost of your slot.
A free player using all is gold to upgrade slots, can get to 10 slots pretty quickly.
Some info here
http://forums.nextgames.com/walkingdead/discussion/1636/survivor-slot-cost-and-free-gift#latest