In Horde, the Survivors get slowly stronger and build up over the course of the game, having more and more resources at their disposal as turns tick down. Due to the way the Horde works, the zombies don't have the same luxury. Also, other than changing the number of cards in the Horde deck or the starting life total, there is little once can do to smoothly scale the experience for more or less players. And changes in rules are a pain to remember and balance correctly. However, a few cards can be added to the Horde deck that mitigate these issues.
Ideally, the Horde's power would automatically adjust for the turn count and number of Survivors. To imitate this in a smooth fashion, a number of spells have an implicit form of scaling. .
Scaling in Time
A bad card for the Horde is one that is brutal early game but easy to deal with late game. We choose not to put a Call to the Grave in ours for this reason, if it comes out turn 3, the Survivors have little chance to win. A good Horde card is small early game and large late game, but by then the Survivors may have drawn into an answer and won't feel cheated
Big mass reanimation spells form much of the Horde's late game, where a Zombie Apocalypse can bring back ten or more creatures. An Unbreathing Horde can come in as a 20/20 creature late game. These cards are not strong early game. The graveyard may be empty or nearly empty. This lets the Horde ramp up its power level into the late game, as the players do.
A quick not, we choose not to include Soulless One for the following reason: its power/toughness vary every time anything happens, and that is a lot of counting and recounting to calculate its toughness. Unbreathing Horde needs the players to count just once.
Effects that depend on the number of creatures the Survivors control can also provide some late-game punch.
Scaling in Player Number
Cards that read "Each opponent" somewhere in their rules text can be very helpful on this front. Infectious Horror, for example, will drain a 2-Survivor team for 4 life, but a 4-Survivor team for 8. Since life totals are 40 regardless of the number of players, this card is substantially more deadly with more players. Vengeful Dead scales in the same way. Syphon Flesh also gets more powerful the more Survivors there are, giving the Horde an extra token for each player.
I'm of the opinion that Horde should be more difficult with fewer players, and easier with more, but not too much easier. Having enough of these self-scaling cards in the Horde deck makes special scaling rules not as necessary.
Scaling for Comebacks
Every game needs some come-back-from-behind things, and the Horde needs them too. Some cards are very powerful when the Survivors have the Horde on the ropes, but weaker when the Horde is already doing well. Living Death, for example, can actually help an overwhelmed group of Survivors in some board states.
"Win more" cards aren't really necessary for the Horde. A slow, relentless onslaught, wave after wave of zombies crashing against your defenses and wearing you down until all hope is lost, that's what we want. The sudden 'Well, you all lose, this card came out" Horde wins aren't very satisfying. Games should be fun and satisfying to lose as well as fun and satisfying to win.