1. If I remember correctly, current maximum is 200 characters. You start with 50? slots, and since it's 1 gem for 5 slots - 30 gems for maximum capacity. But unless you're whaling (or really lucky with gacha), you won't need max anytime soon. After half a year of playing I'm sitting at 75 slots, with ~20 taken by 4* and 3* knights I could get rid of since I don't even use them anymore.
2. No, no bonuses for whaling.