Please note that I'm not an English teacher.
I think your writing is sufficient. I was just reading Montaigne and from what he says, a good communicator does not pay so much attention to the language than to the ideas he's expressing. Since you're expressing yourself in a way that's understandable, I believe your English is adequate. Don't be so hard on yourself!
To answer your questions:
1. I believe that "the number" does not paraphrase "the amount." It may be a different way of stating "the amount," but I do not see the necessity of using "the number" over just writing "the amount." Maybe I could help you if you gave me some kind of context.
2. Writing "the number" implies that you're about to discuss an abstract digit in your prose. When you write "the amount," your reader may anticipate the element of addition or subtraction to be included in your prose. There is no negative connotation to either, but you should use your wording wisely.
3. You should never aim to decorate your writing. There are such things as style and voice, yet those concepts should come naturally. Instead, I stress that you only elaborate when necessary.
"Could I use the adjectives of higher, larger, bigger, greater, more enourmous, more gigantic, lower, smaller and shorter in the abovementioned both sentences?"
You may use any of those in your two sentences below. They will help your reader, and I recommend the use of such adjectives in your sentences.
Student or Learner