I have two files, People and Agencies.

Each Agency can "have" infinite People, each Person can "have" only one Agency.

This means that the actual connection field is in People (Agencies.IDNumber is a field there).

How can I count in Agencies how many People are connected to it?

That is, in Agencies I have a portal that shows the staff members there, anyone who has that agency indicated in their own file. Zero or one or five or twenty.

How do I get that number and use it in calculations?

Create a Count( rel::id ) field in Agencies.

Well, there's quit likely also a 'connectionfield' in Agencies, presumably you called it IDNumber as well.

If you define a relationship from IDNumber in Agencies to Agencies.IDNumber in the People file, you can use the Count function in a calculated field to count the number of related People

The calculation would have to be something like

Count (relationship::Agencies.IDNumber)

Hope this helps,

Ernst.