まぁ多分こんな感じか?
IEnumerator GenerateIns(float time,int count,GameObject obj){
float interval = (float)count/time;
float pastime=Time.deltaTime;
while(count<0){
if(pastime>interval){
Instantiate(obj);
count--;
pastime-=interval;
}
if(pastime>interval)contine;
pastime+=Time.deltaTime;
yield return null;
}