Can I use only Amp instead of mobile theme +Amp , Any SEO impact ?

Hi,

Yes, you can use only the AMP option -> https://www.screencast.com/t/a3UFnyMy from the mobile theme, there will be no problems.
Mobile Theme: Automatic Optimization for Mobile with Newspaper Theme -> https://tagdiv.com/mobile-theme-automatic-optimization-for-mobile-newspaper-theme/
How to Make an AMP Ready Website with Newspaper Theme -> https://tagdiv.com/amp-newspaper-theme/
If you want the responsive version and amp you should use this opinion -> https://www.screencast.com/t/jZct5ueBqOrb but if you have amp activate on pages you’ll need to create in Mobile editor a content for AMP version of page.

Thank you!