How do you use a few more and much?

"A few" and "much" are estimates which people use to indicate either a little or a lot of something. Generally, the listener will have a rough idea, from the context the words are used in, how many the speaker is referring to.

"More" is only used relatively. There must already have been mention of either an exact number or an estimate to which this refers.
If I say there were a few people in the room when I entered, then you will probably be able to guess from what I will have said previously about either the size of the room, or the type of event which was happening, how many people I mean by a few.
But, If I said there were more people on the plane, then you need to know what I am comparing this number of people to.

"Much" is most-often used in phrases such as "too much" - meaning overfull, or more than sufficient, or in a question "How much....?" asking for a quantity to be named by the person beng addressed.
It is also used of uncountable things.... there was much rain yesterday.