Hi Dina,
Concerning the requirement for witnesses, it says on page 4 of the ADRT form on the Mnda's website (link provided below) that one witness is sufficient but preferably there should be two (and as already mentioned by Ellie, they can't be people who stand to gain from your death).
http://https://dbsy278t81889.cloudfront.net/app/uploads/2015/04/19135922/my-adrt-blank-interactive-form.pdf
Further information about ADRT (including what information needs to be included on the form) can be found on pages 65 to 67 in Section 9 of the MNDA's 'End of Life Guide'.
Link to Section 9 (Advance Care Planning and Advance Decisions) of the MNDA'S 'End of Life Guide' provided below:-
(Warning: some people might find some of the information included in the 'End of Life Guide' upsetting)
http://https://dbsy278t81889.cloudfront.net/app/uploads/2017/05/19140014/eol09-advance-care-planning-and-advance-decisions.pdf
Love Kayleigh x