With less than 20 players, any group of humans bigger than 7 or so will be able to handle the whole Horde effectively, even if everyone else has been turned. I'd say probably kick the stun timers up, and structure 3 objectives over the game time in order to force the humans into separate groups. So objective one might be four different capture zones that need holding, objective two a simultaneous VIP escort and item retrievall, then the final objective (when you get down to about 5 humans) a nuke-prime (stand and hold) followed by an exit sprint. With that setup, presuming 20 total players, I'd have two starting OZs, each allowed 2 tags before reveal, or three with one tag each. Perhaps even four, with one placed at each different capture zone.
I'd start with a 5 min timer, going down to 3 after the capture zones objective is completed, then going down to 2 after the VIP/item retrieval was completed, then down to 1 minute for the final stand/sprint.
Capture Zone Objective : Define four 10x10 feet areas as far apart from each other as possible on your field. Have humans take and hold each of them for a set timer (probably 15 mins), but this timer is reset if an unstunned zombie manages to get inside. Revealed OZs count, un-revealed do not.
VIP escort objective : Have a VIP spawn at one edge of your playzone. Players must escort him or her from one edge to the other, but he only moves slowly. If tagged, he must be supported, either by one player using both hands, or two players using one hand each. If tagged again, the objective is failed - either trigger a penalty (stun timers reduced, special Z spawn etc), or reset him to where he spawned.
Item Retrieval : Give an item to a zombie. This zombie may not run(only walk), or hide, but when stunned must drop the item where they are standing. Other Zs may pick up the item, but are then subject to the same rules. Humans start at a capture zone as far away from the item spawn as possible, and must return it to the capture zone. Additional rules (if humans are doing too well), include penalties for humans carrying the item, such as not being able to fire, or being forced to walk.
Final Stand : Humans are given a single stationary item (think laptop/laptop bag etc) to defend, and 1 person must be "interacting" with the "bomb" for 2-3 full minutes (if they fire or move the timer restarts). Once this is done, the humans have a small amount of time to reach a defined exit from the field to win the game. (Maybe 30 seconds? Set Z stun timers v. low for this last sprint.)