suppose that we have a equation of    f(x) = x³-3x²-2x+1

for finding max. posiive roots  we see the change in sign 

means  here =+ to -,

and than - to +

 so two change in sign  so at most 2 positive roots.

for -ve roots put x = -x

f(-x) = -x3-3x2+2x+1

so change in sign -ve to +ve

so at most one negative roots.

so complex roots = degre of equation - (+ve roots) - (-ve roots).