It would be helpful to know the route you are looking at. For many domestic routes between larger stations, only one stop is permitted and trying to add more than that will create a broken fare and you'll end up paying multiple fares. Further, the cheapest fares often prohibit end-on-end ticketing and this will bump you into a higher fare class when trying to book the broken fares as a single ticket. For smaller, less well served stations, they may allow up to two connections. International routes will typically allow more than this. ITA Matrix advance routing codes can be used to specify number of segments (i.e. DL DL DL for 3 segments on DL).