And the answer is "D" - Not enough Information...
Well, there is - provided that it is a medium - you can mix however much you want. Mediums exist on their own as fully functioning paints, varnishes, putties or whatever.
However, ignoring that part for a bit, it is really important to specify. The matte medium that I use for making paints flat is Golden Super Loaded Matte Medium. As the name says - it is super loaded with matting agent (about as much as they feasibly can put into suspension without calling it matting agent). It is thicker though - so not much help for thinning out paints...but a little dab will take a gloss paint to dead flat.
The matte medium I use for clear coats (when not using Dullcote) is less flat. In order to actually flatten out sheens...I would need significantly more of it.
You also don't mention your end goal. If I wanted to go from gloss to satin - it would need a different amount of matting agent than I would need to go from gloss to flat or from something that is supposed to be flat (but isn't quite flat enough) to dead flat.
So...there you go, however it seems as though that isn't really the question you should be asking anyway...
I'm using a cheap paint that doesn't like to finish all one color. You can visibly see brush strokes in the finished product. I am trying to get a solid finish.
Matting agent won't actually help that issue. If you are seeing brush strokes (actual high-low areas where the brush ran) you need a flow aid in order to get the paint to settle down and maybe a drying retarder in order to give it enough time to settle down. Rapid City tends to be pretty dry and water based paints dry pretty quick. Little bit of both mixed in when thinning out the paints may help that problem.
The other possible issue would be if the paint doesn't have good coverage. This can happen with some cheap paints and even some expensive paints when dealing with certain colors (yellows and reds in particular). In order to deal with starved finishes - thin out the paints a bit and do multiple coats of the same color. It can help to switch the direction you are brushing in order to criss cross the brush strokes to get better coverage between coats. Using a white primer under those problematic colors helps too. Yellow on black primer painted by hand is a nightmare even with high dollar paints. Several coats and you may still have a bit of dark area thumbing its nose at you.