I want to have the formula numbers of cases environment flushed on the
right like for regular formulas (not the default
numberdistance=2.5\emwidth). How can I do that?

\starttext

\startplaceformula
    \startformula
        \startcases
            \NC x = 1 \NR[+]
            \NC y = 1 \NR[+]
        \stopcases
    \stopformula
\stopplaceformula

\stoptext