There is not a specific expression for that, but you can calculate the exact number using events.
The number of line wraps is equal to the textHeight
of the full text (line height offset excluded) divided by the textHeight
of a single character / line.
Here's a working prototype: dropbox.com/scl/fi/jmccjnkc9tdvmhchvl8qe/Line-wrap.c3p
Note that modifying the text appearence at runtime is needed for the calculations to work.