NW3.75 possibly
It's quite thin already which probably means there isn't much density left to lose.
I think sorting out the top half of your head would not look abnormal, even if the crown starts to receed more. Hopefully you're on hair loss medications to slow that down.
For a lot of people I think more than 1 HT is likely to be required, it's just a question of when really. 5 years, 10years, 15 years?! Who knows.