duck

float numberOfConcurrentItems = 100;
float lifeSpan = 5;   // in seconds
float spawnPeriod;
float nextEmitTime;

void Start() {
    nextEmitTime = Time.time;
    spawnPeriod = 1.0f / ( numberOfConcurrentItems / lifeSpan );
}

void Update() {
    if (Time.time > nextEmitTime)
    {
        nextEmitTime += spawnPeriod;
        Emit();
    }
}