How to find the num of lines that are parallel to2x+6y+7=0 &hav an intercept of length 10 betn coordinate axes?

For any line there are an infinite number of parallel lines. It is not clear what the second part of the problem means. I think you mean the distance between the x-intercept and the y-intercept is 10. For any given length, there are only two such parallel line with a given slope -- reflections of each other across the either the line y=mx or y=-mx. If this is what you mean, those two lines are the ones you are looking for.