In human development, how many totipotent cells are created in the zygote?

What is the inhibiting agent that prevents the other totipotent cells
from becoming complete human beings?  (i.e. why is the earth NOT full
of conjoined twins)?

When performing embryo cryopreservation, how many cells does the embryo consist of?

This is somewhat of a tricky question.  At approximately the 16 cell
stage there is a differentiation event, in which some cells are
directed to become the trophectoderm which produces the chorion and
the cells of the inner cell mass which will give rise to the embryo.
This is the first differentiation event. So, at this point, no cells
are left totipotent.  However the ICM cells are pluripotent and able
to form ever cell in the body. the ICM consists of approximately 13
cells by the 64 cell stage. The cells in the ICM move down
differentiation pathways guided by Notch/Wnt/Frz/Oct signaling which
serves to inhibit and excite certain characteristics in neighboring
cells.  This sort of lateral inhibition is common in differentiation. 
If memory serves embryo cryopreservation typically occurs 1-3 days
after fertilization so you'd be looking at maybe 4-8 cells.  I'm not
an embryologist, so thats the best I can do.

Good Luck.
PForcelli.