If you hit 1 unit or 15 units, you kill 1 unit with the nuke.
And I’ve nuked colonies to 0 workers before. It’s extremely hard and honestly, the person doing the nuking loses MUCH more in resources trying to keep a person at 0 workers than the person being nuked (as they’ll get more workers from base growth every day). In order to nuke someone to 0 workers, you usually need to have double nukes (launch with 24 tick delay and launch again) in a constant stream hitting every 4-5 ticks for about a week.
That alone (and this is if you want them to be 0 workers SLOWLY) costs 300 energy a day and 600 metal. So that’s a lot of converting because god knows you don’t get that energy income in a day.