So first Marty accidentally goes back to 1955 (1), spends some time there, and goes back to 1985. Then Doc comes and grabs him, and they go forward to 2015. Biff goes back to 1955 (2) to talk to himself, and then returns the DeLorean to 2015. Marty and Doc head back to 1985, find out everything is messed up, talk to Biff, and then head to 1955 (3) to fix everything. They succeed, and are about to hit it back to 1985 when Doc vanishes after being hit by lightening. Then Marty gets a letter revealing that Doc wound up in 1885, and he left the DeLorean in a cave until Marty received the letter, again in 1955 (4).
So that means in 1955 there’s four DeLoreans kicking around somewhere, as opposed to 1985, where they always mostly seem to arrive after they left (minus the first movie where he arrives 10 minutes before), but perhaps exceeded by 2015 where Doc apparently did a lot of mucking about. It is also worth noting in 1885 there are two DeLoreans, from Doc originally going back and hiding one in a cave, and one when Marty went back to save him. There may or may not be even more than all of this because Doc also seems to have been to a lot of different eras in order to gather all that money for his briefcase-o-bills.
