struct SpriteNode{ //simple singly linked list, consider using a standard library one, I just did this to be explicit
    SpriteNode* next;
    int idx;
}

int peakCapacity=0; //represents the last element of the array that has been populated
Sprite sprites[MAX_SPRITES];
SpriteNode* freeList; //this is the first element of the freelist, no seperate head. null initialized

//Don't add initially empty slots to the freelist, we keep a freelist to keep fragmentation low.
//We always want the freelist to be the best choice for assigning to a new element
void init(){
    for(int i=0;i<loadCount;i++){
        //just do whatever normal init you'd do for your sprites array
        sprites[i]=loadNewSprite();
        //then increment the peakCapacity so we keep track of where the iniitalized sprites end
        peakCapacity++;
    }
}
//This must be called whenever a sprite is considered an invalid entry in the sprites array.
//for example your sprite dies. Wherever you would normally set is_used=0 basically
void deadSprite(int idx){
    SpriteNode* tmp=new SpriteNode;
    tmp->idx=idx;
    tmp->next=freeList;
    freeList->next=null;
    freeList=tmp;
    return;
}
//called when we need to introduce a new sprite to the system
//it returns the position where the calling code can introduce their new sprite
int newSpriteIndex(){
    if(freelist){
        SpriteNode* tmpnode=freelist;
        freelist=freelist->next;
        int ret=tmpnode->idx;
        delete tmpnode;
        return ret;
    }
    if(peakCapacity<MAX_SPRITES){
        return ++peakCapacity;
    }
    return null; //we didn't have any free slots on the freelist or the array,
                 //we've reached capacity with no holes.
                 //you should probably increase the array size if this happens. The freelist remains fine because we made it referential by index not pointer
}

//This would be update/render code
//notice how we don't loop through the entire array, we only go as far as we know there's sprites.
for(int i=0;i<peakCapacity;i++){
    //do normal sprite-y things

}