35 is the max, so I think it's a new slot every 10 levels starting at 100. I hit level 100 earlier and am still at 30 max.
If you want to farm pumpkins, I'd recommend ?-3. I've been getting exactly 250 per 40 stamina (500 per 80), with a chance for 260 if I slowed down my squads even more. It seems the glitch has been fixed as I broke 253 a while ago and didn't get any error message.