Will I lost a giveaway slot if I delete a giveaway with 0 entry to edit some settings?

Yes, still, with the giveaways you have created, you have still plenty of slots.

Or you could wait till the giveaway finishes, and then create it again

Yes but don't worry. Number of slot grow up with.. time or level i don't really know
I have more than I need (283 actually), despite the fact that I have deleted so many because I'm not vigilant enough when I create them, but only 30 seconds later so it shouldn't be a big problem for you either.

I just leave private giveaways running and end on their own time.

With Group and Public, yes, deletion removes slots.

For whitelist and invite only GAs you can also use The perpetual I-need-5-entries-for-a-giveaway thread.

Every 3 completed giveaway gives you +1 slot. Each removed giveaway gives - 1.
All in all, if you can't consume it for a bad day, I don't think it's going to be a problem.

It's always worth it to know that in case you need more giveaway slots, (e.g. you're making a big train, or accidentally locked yourself out of making giveaways), you can always write a ticket to support and ask for some giveaways slots.

Yep, and this is a quite obscure rule. I've lost about twenty slots once due to a removing of a badly constructed train.

Thank you all for the help. I will probably close the thread now that my question has been answered. Have a nice day.