The current maximum slots is 20. (1.5 update)
| Level | Gold Cost |
|---|---|
| 5 | (inital) |
| 6 | 50 |
| 7 | 100 |
| 8 | 150 |
| 9 | 200 |
| 10 | 300 |
| 11 | |
| 12 | 400 |
| 13 | 500 |
| 14 | 650 |
| 15 | |
| 16 | |
| 17 | 650 |
| 18 | 650 |
| 19 | |
| 20 |
Some info here
http://forums.nextgames.com/walkingdead/discussion/1636/survivor-slot-cost-and-free-gift#latest