Let's look at the first equation.
3y = 5x - 1
y = 5/3 y - 1/3
This line has slope 5/3. A line perpendicular to it has slope -3/5.
The second line is y = -3/5 x + 9.
Its slope is indeed -3/5, so the second line is perpendicular to the first one.
There is an infinite number of lines perpendicular to any given line. You concluded correctly that the two lines in this problem are perpendicular based on the fact that their slopes are negative reciprocals. The second line, y = -3/5 x + 9, is only one line that is perpendicular to the first line. There is an infinite number of lines perpendicular to the first line. All the perpendicular lines have the slope -3/5 and different y-intercepts. The +9 here is just the y-intercept of this specific perpendicular line. Since there is an infinite number of y-intercepts, there is an infinite number of perpendiculars.