There are two timelines that interact with the 'normal' one: the future Trunks comes from and the future Cell comes from.
Wrong, there are three known alternates.
The one Trunks comes from, aswell as the one Cell comes from. However, there is also the one that the Trunks from Cells dimension went back to.
Cell stated he killed Trunks before he went back the second time (which is why Trunks knew where Cell was that one time, after he had returned), which means that there is ANOTHER dimension, which is the one THAT Trunks went to.
Hence, three other dimensions, including the one that is called the "Original", even though its not proven if that IS the original. There are probably many other dimensions. Most people believe alot of the movies take place in these other dimensions, as that makes sense because most of them don't fit into any timeline, and most of them are standalone stories.
The only one that they said took place in the story was the Dead Zone movie, with Garlic Jr.