Is possible to distribute app only to one account? I'd like to ask you if it's possible to distribute my iOS app only to certain users/client. 

If yes, how?
Yes. Sign up with apple for their custom B2B store and you can sell apps to only the clients you approve.
Your clients will sign up for VPP and you'll need to set  things up when you upload your app to the App Store.

http://www.apple.com/business/vpp/
For the administrative hassle, you get the ability to sell purchase orders, set custom pricing, negotiate custom contract directly with your customers and have Apple still do the app review, the QA and app distribution for you.


Alternatively, your clients can set up a MDM and custom App Store and you can follow Apple's instructions for app distribution outside the App Store.