How to know enemies' health?

Once the console is brought up (using ~ (`) on PC, Alt+5 or Option+N on MacOS), you can use your mouse to select currently visible entities.
As targeted commands control the active entity, always be sure you are targeting the right object or NPC, as entities overlap at times and it's hard to tell which one is active.

To get an NPC's Health, select it and use the command getAV Health.

Update: The easiest way to see enemies health is to install these mods

https://www.nexusmods.com/skyrimspecialedition/mods/12688

Another way is to use console but just check that mod above.

So, a dragon have 3k health. To get 100 one hand skill I will need like 500k xp. So around 100 dragons with normal bound sword.

Another way

I figured that out. Basically, press ~

After that I can click on the dragon and it'll show some numbers

There you go.