I have seen this sentence as, "Looks like there's only one other doorway. " (Does it mean that there are two doorways? )
People might have come across five doorways that did not meet their expectations (too small, too narrow, too dark) and remark that there is just one further doorway to look at:
Looks like there's only one other doorway (that might be a suitable entrance).
In the same way, people often remark:
There's only one other possibility.
That doesn't mean that there is a total of two possibilities. It just means that all other possibilities have been excluded. In this sense, it's the same as saying: There's only one more possibility.
Yes, it typically means there are two doorways.
You would not need to specify one other doorway if there were only one at all, also, you would not say one other doorway if 6 total doorways existed.
Imagine you are lost in a big castle. You enter a room [through a doorway]. Trying to figure out where to go, you only see one other doorway, meaning there is only one new direction you can go. This is based off of the assumption that you are not trying to go back through the door you just came out of.