How to continue \startParagraphNumbers in ConteXt

You can use the \setupparagraphintro to insert a number at the begin of each paragraph. With the reset keyword you can disable the insertion of the number.

\definelabel[parnumber][text=,headstyle=]

\definestartstop
  [ParagraphNumbers]
  [before={\setupparagraphintro[each][\parnumber~]},
   after={\setupparagraphintro[reset]}]

\setupindenting[yes,medium,first]
\setupwhitespace[medium]

\starttext

\noindentation
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc feugiat in sapien nec efficitur. 

\startParagraphNumbers
Donec at mauris ut orci consequat vulputate. Proin vestibulum magna commodo, tincidunt nulla non, fermentum risus. 

Nulla vitae ornare tortor. Suspendisse potenti. Sed sit amet feugiat orci, quis elementum orci.  
\stopParagraphNumbers

\startitemize[2,packed]
\item Nam ut ligula justo.
\item Integer a lacus fermentum, tincidunt libero id,
\item blandit sem. Sed gravida mauris ac sem finibus, 
\stopitemize

\startParagraphNumbers
ac suscipit diam dignissim. Vivamus tristique tempor nisi, eget vehicula velit maximus eu. 

Phasellus vel ornare est, sit amet volutpat nunc. Donec est nulla, porttitor a convallis vitae, pulvinar ut ligula. 
\stopParagraphNumbers

\stoptext

I probably shouldn't answer this since I never use ConTeXt, but what about:

\def\pauseParagraphNumbers{\xdef\savedParacount{\Paracount} \par\endgroup}
\def\resumeParagraphNumbers{\par \begingroup \let\Paracount=\savedParacount \appendtoks \Paragraphnumber \to \everypar}

Which you can use like this:

\definepapersize[pb][width=8.5in,height=13in]
\setuppapersize[pb]

\setuppagenumbering[alternative=singesided,location={footer,right}]

\newcounter\Paracount
\setcounter\Paracount
\def\Paragraphnumber%
   {\increment\Paracount
    \Paracount.~ }
\def\startParagraphNumbers%
   {\par \begingroup \appendtoks \Paragraphnumber \to \everypar}
\def\stopParagraphNumbers%
   {\par \endgroup}    

\def\pauseParagraphNumbers{\xdef\savedParacount{\Paracount} \par\endgroup}
\def\resumeParagraphNumbers{\par \begingroup \let\Paracount=\savedParacount  \appendtoks  \Paragraphnumber \to \everypar}


\definefontfamily [myfamily][rm][XCharter][features={default,quality}]
\definefontfamily [myfamily] [ss][Calibre][features={default,quality}]
\definefontfamily [myfamily] [mm][pagella][features={default,quality}]
\setupbodyfont [myfamily,10pt]

\setupalign[hz,hanging]
\setupindenting[yes,medium,first]
\setupwhitespace[medium]

\starttext

\noindent
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nunc feugiat in sapien nec efficitur.

\startParagraphNumbers
Donec at mauris ut orci consequat vulputate. Proin vestibulum magna commodo, tincidunt nulla non, fermentum risus.

Nulla vitae ornare tortor. Suspendisse potenti. Sed sit amet feugiat orci, quis elementum orci.
\pauseParagraphNumbers

Just to see:
\savedParacount

\startitemize[2,packed]
\item Nam ut ligula justo.
\item Integer a lacus fermentum, tincidunt libero id,
\item blandit sem. Sed gravida mauris ac sem finibus,
\stopitemize

\resumeParagraphNumbers
ac suscipit diam dignissim. Vivamus tristique tempor nisi, eget vehicula velit maximus eu.

Phasellus vel ornare est, sit amet volutpat nunc. Donec est nulla, porttitor a convallis vitae, pulvinar ut ligula.    
\pauseParagraphNumbers

\startitemize[2,packed]
\item Nam ut ligula justo.
\item Integer a lacus fermentum, tincidunt libero id,
\item blandit sem. Sed gravida mauris ac sem finibus,
\stopitemize

\resumeParagraphNumbers
ac suscipit diam dignissim. Vivamus tristique tempor nisi, eget vehicula velit maximus eu.

Phasellus vel ornare est, sit amet volutpat nunc. Donec est nulla, porttitor a convallis vitae, pulvinar ut ligula.    
\stopParagraphNumbers

\stoptext