JOSM can handle 50000+ changes requement and 5000+ changes recommendation automatically

[openstreetmap.org-user-d1g]

Precise logic how to upload 1-5000 changes was covered at https://josm.openstreetmap.de/wiki/Help/Action/Upload#Configuringthenumberandsizeofuploadrequests ("Recommendations")

Also, I don't think we need to ask user to enter at least 2 changesets and provide warnings in 2 dialogues: https://josm.openstreetmap.de/wiki/Help/Action/Upload#Runningaverylargeupload

My idea is that reasonable defaults are better that 4k long manuals everyone have to read. 80-90% of users shouldn't repeat very same task.

"Your changes will be uploaded in X requests, using Y changesets. Would you like to change this? [link]" where X and Y are specified up to "Recommendations" automatically.