In this way, each time a zombie spawn, you'll have to zombies. Then you'll give to each zombies a reinforcement score of 1 so they won't spawn any more zombies. Then you'll use /execute on each zombies that have that score to make them spawn another zombie. Imagine that you set to each zombies that are spawning and score of reinforcement to 2. Now, if you'd like to spawn zombies more zombies even not when you are attacking them, you can copy that behavior with command blocks. In this way, each zombies that you will try to kill will spawn more zombies. So you could use the /entitydata command to increase that stat of each zombies in your game. Each zombie has a 'likeliness to call reinforcements' statistic which ranges from 010, and 'leader' zombies (05 depending on regional difficulty) get a bonus of 5075 percentage points to the stat. This behavior is controlled by the stat zombie.spawnReinforcements. On Hard difficulty, zombies can spawn additional zombies to help when damaged. This means that in vanilla minecraft, when you'll try to kill a Zombie, (if you have hard difficulty on), he may be able to spawn other zombies to kill you.

Both the damaged zombie and the new zombie will have a 5 percentage point penalty to their 'likeliness to call reinforcement' stat, making it less likely that an infinite number of zombies will spawn. When the zombie is damaged by an entity or is damaged while targeting an entity, up to 50 attempts will be made to randomly choose a spawn location (0 or ☗–40 blocks away in all three axes) that is above a block with a solid top surface, has light level 9 or lower, has no players within 7 blocks, and has no colliding entities or blocks at which to spawn the reinforcement.

Each zombie has a 'likeliness to call reinforcements' statistic which ranges from 0–10%, and 'leader' zombies (0–5% depending on regional difficulty) get a bonus of 50–75 percentage points to the stat. On Hard difficulty, zombies can spawn additional zombies to “help” when damaged.

7 days to die do zombies get stronger