That is simple. You can use "multo"multo as a description of a big number of smthsomething countable. For example "multo da tabloj"multo da tabloj. Like you say - ten, hundred, thousand - multomulto. And, sure, multomulto is a noun
"Multe"Multe can be used as a description of something uncountable. "Pluvas multe" Pluvas multe - It"It rains a lotlot". MulteMulte - adverb