How many battery cells in a smartphone?
Yes, most smartphones have a single cell battery.

Most of the components on the smartphone mother board are low powered and run on voltages lower than the typical 3.7V provided by a single lithium ion cell. Hence a series combination of multiple cells is not worth it at all.

Having a parallel combination of cells is also not useful because of the additional complication of balancing the cell voltages. A multi cell system would require cell voltage balancing for the sake of safety and longevity of the cells. 

Also, a balancing circuitry would not only add to the hardware costs of the smartphone manufacturer, but would also make the phone heavy and slightly more expensive. The balancing would also increase the charging time of the phone.

All of these issues with multi cell batteries work in favor of having a single cell battery in smartphones.