I'd say that only one photo can be enough to make a photomanipulation. A face retouch in which the artist painted over and say for example, converted a simple photo of a man into an artwork showing an elf. Then there'd be struggle in making a difference of wether that's a photomanipulation or a digital painting based on stock imagery. I'd say it's both.

Then using only one photo, where's the limit? I do not consider making some colour arrangement/ effects is a photomanipulation in terms of artistic design. The whole point of this is, if you make a creative composition that differs from the original, then that's a photomanipulation. In the previous example, now the artwork is not a photo of a man, it's a photo of an elf, the works are "original" by themselves because they transmit different concepts.