Assuming you have looked at .NH 2 to give you 2nd level numbered headings, and that is not what is wanted, you can use your own number register, as shown in the man page .IP indented paragraph example.
Add to the start the definition and initialisation of your number register (starts at 0, increment by 1)
.nr mytheorem 0 1
then whereever you use it as
\n+[mytheorem]
it will be incremented and replaced by the current value. Eg use
.PP
\*[SN]\n+[mytheorem]
.B Theorem .
where SN is the section number (from .NH).
You can reset the register by giving the .nr definition again.
You can put the above into a macro at the start:
.de myTHEOREM
.PP
\\*[SN]\\n+[mytheorem]
.B Theorem .
..
and call it each time:
.myTHEOREM
Magna dolore.
.EQ
e sup {i tau} = 1
.EN
.NH), the bold text (.B) or the equations themselves (.EQ).