今回は"2017-10-28 17-50-46.jpg"->"171028_1750(_連番).jpg"に使った

@echo off
setlocal enabledelayedexpansion
cd %1
echo 変数名name,^%はエスケープ
echo ~m:m文字目から,~-m:後ろからm文字目から,n:n文字分,-n:最後のn文字分を除く
set /p rencmd=%%name:~2,2%%%%name:~5,2%%%%name:~8,2%%_%%name:~11,2%%%%name:~14,2%%
for %%a in ("*.*") do (
    call :sub "%%a"
)
pause
exit

:sub
set "name=%~n1"
for /f "tokens=*" %%b in ('echo !rencmd!') do (
    set filename=%%b
)
if not exist "%filename%%~x1" (
    ren %1 "%filename%%~x1"
) else if exist "%filename%%~x1" (
    set cnt=0
    :loop
    set /a cnt=cnt+1
    if not exist "%filename%_%cnt%%~x1" (
        ren %1 "%filename%_%cnt%%~x1"
    ) else if exist "%filename%_%cnt%%~x1" (
        goto :loop
    )
)
exit /b