Smartphone Cameras - What's the Optimum Amount of Pixels?

In my unscientific gut opinion, as large as the sensor could go with around 8 megapixels but DPReview has a much better explanation with the following conclusion:

... for most purposes 3MP is plenty, but you might want to shoot at around 8MP for the crispest possible details. 8MP is a sensible minimum for big prints and 4K TVs, and even higher resolutions allow you to crop photos without sacrificing quality too much. However, make sure that very high resolutions are matched by an equivalent increase in sensor size.

However, do note that image quality also highly depends on the lens, image processing, sensor design, etc. so the search for the Holy Grail continues...