Re: how-many/count-matches for non-interactive use

[Richard Stallman]

    (defun how-many (regexp &optional rstart rend)
      "..."
      (interactive
       (keep-lines-read-args "How many matches for (regexp): "))

    How do I use "p" here?

Since it is using Lisp code, you just do it with more Lisp code.  Add
a t to the list in a place that corresponds to where the new extra arg
gets its value.