I was wondering does a 6 in and 9 in round tier cake feed 40 people

Probably not. 

Here's a commonly used cake serving chart:

http://www.earlenescakes.com/ckserchart.htm

6 in serves 8, 9 in serves 22. 

7 in serves 10 and a 10 in serves 30 that would be 40.  This is using Earlene's cake chart.  You can google it for reference.

Hope this helps.

AI'm doing a cake this weekend, the guest list keeps rising but so far it's 30 adults and 30 kids. When it was 50 I was going to do an 8 and a 10 but now I'm wondering if I should go bigger. There will be cake pops there too. Thoughts?