just wondering if 9 units of botox will be enough to notice a difference?

9 units of Botox is about half the normal dose for that area. I have many patients that do low doses of Botox. They understand that it will not last as long but like to have some movement of the muscles. Given your age this sounds appropriate. 

It is difficult to say if you will notice a difference with 9 units of Botox.  The amount you received is very conservative.  If you do not notice enough of a change you can elect to have more product injected after 2 weeks.

9 Units of Botox is not a lot at all. It may work for you and it may not. It will take you around 7 days to notice the full effect. If you still have motion of your Glabella muscle, which you will, you may need more to knock it out completely.