For iOS7, a version that take care that you can have differents font (and font size) in your UITextView :
- (NSUInteger)numberOfLinesInTextView:(UITextView *)textView
{
NSLayoutManager *layoutManager = [textView layoutManager];
NSUInteger index, numberOfLines;
NSRange glyphRange = [layoutManager glyphRangeForTextContainer:[textView textContainer]];
NSRange lineRange;
for (numberOfLines = 0, index = glyphRange.location; index < glyphRange.length; numberOfLines++){
(void) [layoutManager lineFragmentRectForGlyphAtIndex:index
effectiveRange:&lineRange];
index = NSMaxRange(lineRange);
}
return numberOfLines;
}