What do postdoc advisors/hiring committee typically look for in a potential theoretical high energy physics postdoc candidate?
Typically how many recommendations, does "the more the better" hold true here?


The job advert will usually tell you how many references to provide. I'd be wary of exceeding the number they ask for -- you want to demonstrate basic competency by following the instructions about the application. The same goes for any formatting of CV, cover letter or research statement that they require.


  Also does it matter if referees give okayish recommendations but are well recognised versus strong recommendations but not-so-well recognised?


An okayish letter from a well-known researcher is a waste of a letter. You want (at least) three strong letters, ideally from people who have worked closely with you, and on different topics if possible. 


  Is there a threshold to the number of papers published, and typically how many?


Individual people doing the hiring may have their own prejudices, but there is no community-agreed upon minimum number. One high quality first author paper trumps five weak papers where you're in the middle of the author list.


  Also are preprints on ArXiv taken into consideration, if not published in a journal?


If you've submitted the paper to a journal you're probably fine. I can't answer your question about single-authored papers as they are extremely rare in my field (cosmology).


  How important is demonstrating here that the candidate has a well defined career plan ahead?


Writing about your long-term career plans is less important than writing about the research you plan to do in the job you're applying for, and how working at that particular institute, with the particular person who's hiring will enable you to do that research better than if you went anywhere else. (This obviously depends a bit on the job being advertised; some postdoc positions already have a well-defined research plan laid out in the advert, in which case you need to emphasise how your skills and previous experience make you the ideal person to work on that project.)

You may be asked about your longer-term (2-5 year) career plans at interview. This is another opportunity to emphasise how doing this particular job will get you to where you want to be in that time frame.


  Any other relevant factors?


Above all, people want to hire strong researchers, who they know will get the job done. Your strengths are best demonstrated by your published research, but there are other skills which are important for academic success. These are things like verbal presentation skills, the quality of your written English, networking ability (particularly if you can build a network distinct from your PhD supervisor's), organisation and committee experience, teaching and supervision experience (this becomes far more important as you progress in your career)... the list goes on. You need to consider if you have evidence for each one of these points that you can highlight in the appropriate place (CV, research statement, cover letter or interview).

However, I reiterate my comment: talk to your supervisor. They are the person best placed to evaluate your application before you send it off and help you to improve it.