Asthma doesn't affect growth and development. No worries!
You could have a low sperm count, or you could have some more growth. I can't assure you that you'll still be growing - I would either be lying or making a wild guess. Growth and puberty vary so much from person-to-person, and by 18, you're close to ending puberty and it's likely that not much more will happen.
Sorry, I'm only telling you the truth