I was told that a teacher teaches different subjects and an instructor teaches only one subject, is this correct?
No, it is not true. My Teaching Certificate certifies me to teach Spanish. In the past I was an instructor in the Air Force. I went to specialized classes for that but did not have a Teaching Certificate.
The real truth behind all this that synonyms of teach are educate, instruct, school, tutor, coach, train; enlighten, illuminate, verse, edify, indoctrinate; drill, discipline.
The words are bounced around a lot and some people do not personally accept some of these words. For many years while I was teaching I wore a name tag that said "Professional Educator."
This does not cover every English speaking nation, but is more common in the US.