"I do not eat too much."
Translation:No como demasiado.
As far as I understand it, it's already build into the word "demasiado." Hence your sentence would be "I do not eat too much much." Or something to that effect.
I initially did the same translation. However, demasiado actually means too much. Therefore, saying demasiado mucho is like saying too much much. :)
I tried "muy mucho" since a few questions before it told me that also meant "too much" (although I never learned that meaning before) and it marked me wrong. Some of this gets very confusing. Need more input from native speakers :D
"muy mucho" may mean "very much" and that maybe a bit different than "too much". "I do not eat very much" could mean you're starving, underweight and need food. "I do not eat too much" could mean you are in perfect health and not overweight. Though the person may also mean they need food too.
"Muchísimo" is the way to say "very much" in the sense of a substantial amount of something. Examples:
"Lo quiero muchísimo"
I do not know the actual grammar rule, for why it is wrong, but it is, or at least is never used.