There are water sources periodically along the trail up to Trail Camp, so you can get your water every couple of miles up to that point. Reliable Water Sources Along the Mt. Whitney Trail (a PDF document)
  related pictures: Water availability along the Mt. Whitney Trail

However, on your summit day, when you leave Trail Camp and push to the summit, you have to carry ALL your water, since there are no sources above that point. The rule of thumb is 3 liters from Trail Camp, but your experience may vary.

Good luck!!

By the way... October 18 is quite late in the year. You will have cold and short days to deal with. ...and possibly snow on the trail (Do NOT go if any storm is predicted!) I see you are from NM, but still, you can try for earlier. If you go in September, just walk in and pick up a free walk-in permit. These are always available if you go mid-week after Labor Day. You can see this from the Unused Whitney Permits numbers.