\starttext

% has number
\placeformula \startformula
f(x_0, u_0)=0.
\stopformula

% has no number
\placeformula \startformula \startalign
\NC \dot{x}(t) \NC=f(x(t), u(t)) \NR
\NC y(t) \NC=h(x(t), u(t)), \NR
\stopalign \stopformula


% no numbers even here
\startsubformulas
\placeformula \startformula
\dot{x}(t)=f(x(t), u(t))
\stopformula
\placeformula \startformula
y(t) =h(x(t), u(t)),
\stopformula
\stopsubformulas

\stoptext

Regards,
Vyatcheslav