The second formula has NO NUMBER (example taken from the ConTeXt Garden's Math/Display page):

\starttext

\startsubformulas[eq:1]

\placeformula[eq:first]

\startformula

c^2 = a^2 + b^2

\stopformula

\placeformula[eq:second]

\startformula

c^2 = a^2 + b^2

\stopformula

\stopsubformulas

Formula (\in[eq:1]) states the Pythagora's Theorem twice, once in (\in[eq:first]) and again in (\in[eq:second]).

\stoptext