Tokencount(text,newline) counts the number of line breaks, it will not work here.
I don't think it's possible to correctly count the number of lines in text object. You can use Text.TextHeight property to get the text height in pixels. And then divide this number by an average height of each line, say 30px. You'll get an estimate number of lines which may not be very accurate if you have a lot of text. Also, different browsers render the text slightly differently, so the results may vary.
With SpriteFont you can calculate line height and the number of lines precisely.