I'm confused. That you are not using x-values does not seem relevant. You plotted Mc on the horizontal axis.
Next, the line:
Does not create a loop. Well, it does, but the loop has length only one step, at N=8. All it does is assign the value of 8 to N.
Next, you want Mc to lie between 8 and 12.
N = 8;
Mc = linspace(8,12,100);
Pi_c = 1+(2*N-1)*(3-2*Mc/N).*(Mc/N).^2;
Which is only the region you seem to be interested in. We can even add some white space on the axes, to show you that indeed, only the part of interest was created.