Generally, yes, the treatment for morning sickness should be fairly immediate. It may return until it is resolved completely, but somewhere between 4-8 hours after the tx (at least) the patient should be feeling better.
We use some different points for morning sickness, but nothing too far out of the ordinary from what everyone else has recommended. For me, PC 6 appears to work just fine particularly if the patient regularly massages the point during the week.
The only other somewhat unique thing we do on the more recalcitrant cases is tuina massage for the vagus nerve. Essentially tuina in the ST 10, 11, 12 area. This is helpful for many causes of nausea and vomiting, particularly those that don't respond as expected to standard points.
Number of Needles:
There are certainly many theoretical and stylistic needling choices to make. Many of the Japanese styles use very small number of needles, as do many other systems. I know of a handful of practitioners who only use 1 needle per treatment and many others who use 4-6 on a frequent basis. Personally we use between 12 and 20 on most patients but as we focus on the huatuo points primarily we don't feel it factors into the "energetic overload". Needling technique and depth also factors into how many needles you would use. We do use a very small number of systemic/pattern points to send very clear messages to the body on that level - often using ev pairs.
As a side note, we do have a treatment page for morning sickness. If you haven't see the acupuncture protocol page for that, go to the morning sickness page and half way down on the left you will see a link to the acupuncture protocol for that condition. The condition pages are somewhat new and the layout will change, but there may be some useful ideas in the protocol page.