In the case of grep, you can simply use the -c option

dpkg -l | grep -c python

From man grep

   -c, --count

          Suppress normal output; instead print a count of matching  lines

          for  each  input  file.  With the -v, --invert-match option (see

          below), count non-matching lines.  (-c is specified by POSIX.)

More generally, you can pipe command output to the wc command e.g.

 dpkg -l | grep python | wc -l