Well a calculation field with Count ( PortalTableOccurrence::NeveremptyField ) will display your total count of related records--thus the total records in your portal at any given time.
I've used a number of portal filtering strategies to prune down the total number of records visible at one time. (To select another group of records for viewing/editing, you select a different value in a drop down that's part of the filtering or click a button that performs a script that changes a filter field's value.)
In March, 2013 Soliant Consulting published Sara Severson's excellent script for dynamically filtering portal results (capability introduced in FM11) as users type into a global field:
I too had the impulse to display a count of the results, and added two lines to her script to set $$Filtered_Portal_Count as the filter is updated:
Between Sara's two lines:
Set Field by Name [$field ; Evaluate ($field)] // Refresh portal without flicker
Go to Object [Object Name: $filter] // Return to global filter field
Nowadays, such a variable can be merged into a layout without the creation of a field!