There are numerous types of lathe saddle stop. There are two basic reasons for using such devices.
One is setting a stop to prevent accidental excess saddle movement which can result in crashing into the workpiece or chuck/faceplate etc.
The other is to have a stop to accurately set the travel to a specific dimension such as when turning to a shoulder.
Of course a single stop can be used for either one or both purposes. In my case I am more interested in crash prevention because I have a DRO on the lathe and I am mostly happy to use that for accurate length settings. Accordingly, my needs are served well with a very simple device even if it is nowhere near as good for length control as the fancy types with micrometer style adjustment.
Click for full size photo.
As simple as it gets, a split clamp on the lead screw and a 6 mm plate bolted to the saddle.
To use just loosen the clamp and move it to the desired position.