@SarahMurdock Newcrest is empty by default: no sims, nothing built on the lots, just a world for you to do whatever you want. Of course, your existing saves should have your sims in them if you ever added anything to Newcrest in that save. Otherwise, if you've never worked on Newcrest in that save, it will still be empty. Still, if the worlds menu doesn't list duplicates, I wouldn't worry about it.
For your missing saves, it's possible that having too many of them in the saves folder prevents the game from reading them all. If you go into the saves folder, you'll see a number of files named Slot_0000000x.save (x being a digit 0-9 or letter a-f, then it rolls over to 10). There will also be Slot_0000000x.save.very, with the y being a number from 0 to 4. Those are the backups for a given save, the data from up to the previous five times you saved before the most recent one. So you might have six files per save in there, and however many saves you've created.
I'm honestly not sure whether the game will load every save at the same time no matter how many there are. But you can move all those saves to a separate folder (say, saves backup on your desktop), then start adding them back in small groups. Load the game to the Main Menu each time, and see what shows up. Be sure to count the total number of saves available to load—it should exactly match the number of Slot_0000000x.save files you've added back, but it shouldn't count the .verx files.
After you've gone through your saves, let me know whether you're missing any. It's possible they were moved somewhere else on your computer, in which case you might be able to find them.
