". on save get....Database error with no messages/log"
What is the exact text of the error?
The lesson module is a standard Moodle component so if there were a limitation of 999 Lessons per course then there would be a similar limit on other modules e.g. quiz or assignment. I am not aware of any such limitation.
This sort of limitation is vanishingly rare in Moodle and a wild guess is a data field with a maximum size.
If you have the funding you could engage a developer to investigate the issue and if it is a limitation of core Moodle they could arrange to get a change integrated. into to core code. Can you confirm what version of Moodle you are using and that it is not a new limitation, i.e. it is not something that used to work and now has stopped working? If you have access to a developer these are the tables used by the lesson module
The information on what modules are stored in a course is stored in this table
You could also go here
And search on various terms to see if it is a known issue. I have been following this sort of thing for a long time and I have never seen it come up, but then it is unusual to have that many items.