"Depends on how accurate you need to be, but."
It has to be absolutely accurate. It is supposed to be a GUI library, so the user should be able to use any text and it should recognize how many lines the text has. The only workaround so far would be setting the number of lines manually. But that wouldn't work if the text depends on user input.
The only way to find out the height on runtime I could imagine would currently be rendering the text onto a bitmap and examining that bitmap. That would be unnecessarily slow and would ignore empty lines at the end of the text.

"Not sure how accurate it is,..."
Not accurate enough to work for all possible texts.

"ps: the height of the character is the font number i think no?"
Yes, but this includes the space between lines.