\starttext
\section{One}
\startitemize
\item \type{\displaystyle}: $\displaystyle\sum\lim$
\item \type{\textstyle}: $\sum\lim$
\item \type{\scriptstyle}: $\scriptstyle\sum\lim$
\stopitemize

\section{Two}

$\underbrace{xyz}_{n}$

\section{Three}
also posted at https://tex.stackexchange.com/questions/518262/context-number-formulas-side-by-side

\placeformula
\startformulas
  \startformula
    \startalign[number=auto]
      \NC a \NC= b \NR
    \stopalign
  \stopformula
  \startformula
    \startalign[number=auto]
      \NC a \NC= b \NR
    \stopalign
  \stopformula
\stopformulas
\stoptext