Do phospholipids form one layer

No they form a double layer

Explanation: Because the heads are polar molecules they (form hydrogen) bonds with water molecules found outside the cell and in the cytoplasm and they arrange on the outer layers (the outside of each side) of the cell membrane. In contrast the tails are nonpolar and (cannot form hydrogen bonds) are found in between the polar heads in the middle of the cell membrane. The tails bond to each other.
(Phospholipids form a double layer because heads, water loving, are attracted to the water in the cytoplasm inside the cell and the watery fluids outside the cell. The tails are water repelling and they are between the or in the middle of the heads.)