138 rows in a portal is a lot of portal rows.
But the question mark is probably due to the field being too narrow. If you enter layout mode and make this field a little bit wider, the ? result should disappear replaced by the number value.
I've tried expanding the width of the row (field) but still I'm getting the same ? result on every 138th row, below is a snapshot of what I'm getting. Depending on the amount of records in a day, this portal could grow up to 200 rows or more.
And if you click into the field, you still get a question mark?
Then it would appear, I am guessing, that you are using GetNthRecord in your calculation and you've reached a "recursion limit" in system resources.
There are other ways to number your portal rows--such as using Get ( RecordNumber ) in an unstored calculation field. Or even no field at all if you use the Record Number symbol added from the Insert menu.