How to add dynamic number of cells in a horizontal recyclerview that spans to the complete width of

One way is to divide display width by item count.

In your Adapter you have method, from there you get item count

    @Override
public int getItemCount() {
    return your_items.length;
}

and in onCreateViewHolder you can change width of your item layout

 @Override
public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {
    // create a new view
    View v = LayoutInflater.from(parent.getContext())
            .inflate(R.layout.item_view, parent, false);

    //get device screen width
    int width  = Resources.getSystem().getDisplayMetrics().widthPixels;
    //set your item view width
    v.getLayoutParams().width = width / getItemCount() ;

    MyViewHolder vh = new MyViewHolder(v);
    return vh;
}

Not tested, but it should work.