Get a calendar and count them. February always has exactly four weeks, but other months might have four or five Tuesdays and it changes each year. So your answer is "about four".

There are 52 weeks in one year. You can divide 52 by 12, but since this will bring up 4.3 means, as JV said- some months she will have 4 lessons and some months she will have 5 lessons.