"[...] each creature of your choice that you can see within 10 feet of you must succeed on a Dexterity saving throw [...]"

I can't seem to find an answer to my question anywhere. So I try this here.

What's the saving throw DC the target has to succeed on and how's it calculated?

I believe that the saving throw would be the same as your Ki save DC, which = 8 + proficiency bonus + wisdom modifier.

KnightOfTheMop is correct, the Monk class KI entry says:

Some of your ki features require your target to make a saving throw to resist the feature’s effects. The saving throw DC is calculated as follows:

Ki save DC = 8 + your proficiency bonus + your Wisdom modifier

Thanks both of you!