If my oxygen system has more than one oxygen cylinder and they share a common manifold, what will the pressure read? Why does the pressure drop a little sometime after the fill?

Regardless of the size of, or number of, oxygen bottles in your aircraft, if they share the same manifold the system will always read whatever the final pressure is. During an oxygen transfill the tank being filled will heat up. This causes the gas to expand. Depending on the cylinder volume, after it cools back to ambient temperature it will reduce in net pressure about 2% to 5%.