Re: How many people master English language good?
Many people master the English language, and other languages as well. But mastering a language is one thing and speaking it like a native is quite another. It is, in my opinion, very difficult to speak like a native, but not impossible. Accent is just one aspect of speaking. Speaking like a native also involves the right intonation, a (more or less) vast vocabulary, the correct usage of phrasal verbs and idioms, and some (probably many) other minute aspects/details of speech.
Please be aware that I'm neither a native English speaker nor a teacher.