When object A hits object B number go down

If you post a code snippet, ALWAYS USE CODE TAGS:

How to use code tags: Using code tags properly

Definitely start with some tutorials on collisions and/or collections. There’s plenty!

Remember: code is only a tiny part of the problem. The much bigger part is setting up the scene properly.

How to report your problem productively in the Unity3D forums:

http://plbm.com/?p=220

How to understand errors in general: