There is a sweet spot edk. Beyond a certain point, the increase in oxygen transportation that an elevated haematocrit brings is overshadowed by the increased viscosity. Too many red blood cells makes the blood thicker, like a 'sludge' and provides more resistance to flow.
If you want to get complicated, resistance to flow is calculated by the equation: R = (8LV)/pi*r^4 (where L = length of the tube (i.e your arteries/veins), V = viscosity and r^4 is radius of the tube to the power of 4).
Total flow rate is calculated by F= (change in pressure) / resistance.
Look up the poiseuille equation if you are interested in learning more, as this equation combines the two.