Non native speaker here.
"Much" refers to something not countable, like "much more milk", and is used without plural, "liter" is countable such that you could say "many more liters of milk" (note the plural of liter). So while I'd say the latter is more correct according to grammatical rules, if both are common, both can be considered correct, as rules are often rewritten to adapt to the way the English language is used.