I'm using {{FoundCount}} to get count of filtered records in a portal. This works well when just displaying the count on the layout but I'd like to use that figure as output to a field (regardless if this needs to be a calculation field or just number). How do I have a field track count from a portal?

Is your portal filtered on its own? If yes, can the filtering be reproduced by using a relationship only?

In general, portal filtering works on layout-level only, and it's difficult (though not impossible) to get the result at the data level (where calculation fields operate). If you really need it at data level, you probably don't want to accomplish it using layout-level measures, because that will only work on that specific layout, and depend on that layout remaining unchanged.

I am agreed with comment, use a relationship and then on the parent table, make a calculation field with "Count(field)" function which counts the field from the child table (the one used for relationship, this can be your filtering criteria). If you need more than one filtering criteria, then add more TO of the child table and use a new field for the relationship.

Excellent! Thank you both