Resolution:
The count can be accessed via the following syntax:

- Concat the characters ".#" to access the current absolute count (w/o regard to filters). For example, if you have a tree with the path a.b.c, where b is the repeating element, use a.b.# to get the current absolute count.
- Concat the characters ".##" to access the current filtered count. For example, if you have a tree with the path a.b.c, where b is the repeating element, use a.b.## to get the current filtered count.

Please refer to Chapter 5 of the MB User's Guide for further information.

In MB 4.x you won't be able to Map a field from input side to do above stuff - so you just need to Click on Edit Formula and type the whole formula.