Flexible Metal Conduit length limits with grounding
I am confused as to whether you are using FMC or MC cable. 

Flexible Metal Conduit

This is empty conduit (a pipe) in which you add your own wires.  The metal shell of FMC can be used as a grounding path provided it is less than 6 feet and terminated into metal boxes with cable clamps listed for FMC.  But I don't consider it a best practice.

You add as many THWN-2 (aka THHN) individual wires as you please.  For circuits >20A, you can run currents out of the 75C column of 310.15B16, so in this case #8 is ok for 50A.  

Some people have only ever seen Romex style cable in their whole lives, and have never conceived of the idea of individual wires.  Those people believe when you use conduit, you must then chicken-choke the very stiff Romex down the conduit. However, since Romex comes with a bare ground wire, I doubt you'd be asking about grounds in that case. 

I recommend you run a ground wire in FMC regardless.  If you don't want to buy yet another color, then simply strip the insulation off; bare is an acceptable ground color. 

MC Cable

MC cable has all its wires pre-packed inside the MC cable.  The shell may not be used as ground, period.  Proof of this is that they provide a green or bare wire for the specific purpose of being a ground...

...which begs the question "Why would you need shell as ground?"  If the answer is "Because I plan to use green as a conductor", nope.  You cannot re-task or re-mark a green wire to be anything but a ground.  For that, run 12/3 MC; or run FMC conduit and any wires you please.