which is the standard for 9mm pistols? 9x18 or 9x 19

9X19

The 9x19 also known as the 9mm Luger.

9x19 is also known as 9mm Luger, 9mm Parabellum, and is what most people think of when you say 9mm. This is what Glock, Sig, and virtually every manufacturer chambers for these days when you get a 9mm.

9x18 goes with Makarov pistols, and perhaps a few others, Russian/USSR or Chinese manufacture most likely.

Yep, everything above is correct. Buy the proper ammunition for the gun you have. Happy shooting!