This only applies to normal dynamic textfields, not TFL. These have their own methods to center text both horizontally and vertically... much easier.
There's a property called numLines, but this only states that the chosen TextField will have an x number of lines that you specify, you can't use it to return the number of text lines that you see.
So you have to make another type of calculation, and the textfield must be multiline.
var myHeight:Number = myText.height; //store the height of the textfield
myText.multiline = true; //don't need to do this, if it is in the properties panel
myText.wordWrap = true; //same as above
myText.wordWrap = true; //same as above
myText.height = myHeight; // here is the trick, try removing this line
trace("DYNAMIC TEXTFIELD HAVE "+myText.maxScrollV+" LINES");
Cheers