> Isearch could show in the mode line how many matches follow or precede 
> the currently highlighted one (or we could write "3 of 4 matches", as, 
> e.g., Firefox does).  For big files, it could be helpful to calculate 
> this information either lazily or in another thread.

Lazy-counting is now pushed to master.

> If we had this, it would also be convenient to say "go forward n
> matches", so that typing "M-3 C-s" during a search would be the
> equivalent of typing C-s three times.  To do this we could add numeric
> prefix arg handling to C-s/C-r/C-M-s/C-M-r, if that isn't already
> taken for some other behavior.

Adding a numeric prefix arg to C-s/C-r is implemented in
https://debbugs.gnu.org/29321#86

But this implementation causes flicker because it updates
isearch highlighting on every search hit while traversing
all found matches COUNT times until it reaches COUNT-th match.

Attached is a better approach that adds the COUNT arg to
isearch functions down to the lowest-level isearch function,
thus making it compatible with the COUNT arg of re-search-forward: