I have been asked by sales to create a field on an Account that counts all the opportunities associated with that Account. They wish to use this in various reports, among other things.

For example, if an Account has 4 Opportunities in its Opportunities Related List, this field would have the number "4" in it. And "0" if there are none.

I have been told I can do this with a cross-object formula, but cannot find any help or existing entries here regarding it.

Has anyone tried this and/or have some idea on how to do this?

Thank you.