This is a way to spawn another mob if an existing one dies:
Player Event: On Join {
Variable: = (Set variable "mob_alive" to "false")
Repeat: While = (Put the variable "mob_alive" along with the text "false") {
Control: Wait
Game Action: Spawn Mob
Variable: = (Set variable "mob_alive" to "true")
}
}
Entity Action: On Death {
Variable: = (Set the variable "mob_alive" to "false")
}
This example spawns a mob while the variable "mob_alive" is false. So, while the mob isn't alive, it will spawn a new one.
I hope this helps!