Septomom!

superstar embryo with 8+ cells

Yep!  3-days after fertilization 7 out of 8 eggs made it to the next phase.
7 embryos....dividing wonderfully.   Six of them have 8 cells and one embryo already has more than 8 cells (like any other living organism, if all goes well, they'll continue dividing into millions of cells that will eventually become specialized over the months, some of those cells forming the heart, others forming toes, some the elbow, and so forth).  If for some reason, something goes wrong in the growth phase of each of these embryos, the cells will receive a chemical signal to stop dividing, and that would be the end of life for that embryo.  It's kind of up to God at this point...science can only take you but so far.

The next stage in this IVF process is to determine what day to transfer the embryo(s) back into the womb and 'how many'.  The most advanced fertility centers are generally very conservative in their approach.

The first goal is to select the best-performing embryo as it will have the best chance at leading to pregnancy.  The second goal is to prevent multiple births as much as possible -- so transfer the least number of embryos possible.  Having unexpected twins/triplets/quads, etc comes with its set of medical, financial, and psychological burdens on families.

When embryos are transferred on Day 2 or 3, it's tough to select the best embryo as they may all be dividing at the same rate.  On Day 5, they would have reached the blastocyst stage and would already begin to differentiate between placental and fetal tissue --- then it's easier to tell the superstars from average 'embies'.