There is no predetermined limit as to how many histograms one can use in a sequential fit. There is a limitation based the amount of memory and swap space available on your computer, but before that limit is hit, you will see the speed of refinements (and everything else) on your computer start running very slowly, as in order to complete work, the poor computer needs to move information in and out of its memory to “disk storage,” which these days is probably also solid state memory, but just slower. However, GUIs start getting clumsy when handling long lists. I have heard from people who perform very large sequential fits, with multiple thousands of histograms. They find that much easier to do with scripting. Where the transition point between the GUI and scripting will be is somewhat a matter of taste, but probably somewhere between a few hundred and a thousand histograms. Scripting is outside the scope of this book, but there is a GSAS-II software manual with scripting code examples found at URL https://gsas-ii-scripting.readthedocs.io/ and there is a tutorial on scripting at URL https://advancedphotonsource.github.io/GSAS-II-tutorials/PythonScript/Scripting.htm.