Dear Student,
When we are standing on the floor, then our weight is in the downward direction, due to the gravity, and as per the newton's
3rd rule, every action have equal and opposite reaction, so here reaction force will be in upward direction, which will balance
the gravity. Hence
Mg = N
M is the mass of the man,
g is the gravity
N is the normal reaction of man due to the gravity
Hence the value of upward force will be N, means N = Mg,
It depends on the man's mass and gravity.
Best Regards