No, unless you save a world on your computer files
Yes. The files save locally, and you can have multiple save files in one computer.
First, go to the Minecraft folder. Go to the save files. Copy the world you want, and paste it back in. This will copy the world, and make a duplicate.
If you are playing online, it will save the world saves to your hardrive, and if you are playing on the downloaded version, it would depend on where you saved the file. If you saved it to the hardrive (which is default) it will save the world files to the hardrive, obviously. If you saved it to a flash drive or removable storage, it will save the world files to that instead.
Locate your Minecraft folder, then find your save file. Copy and paste this onto your hard drive. Minecraft is installed in your AppData folder (C:\Users\[username]\AppData\.minecraft\) The saves folder should be in the .minecraft folder.
Once you save data to a CD and burn the disk you must open the file(s) that you want and re-save them to your hard drive. Then you can make changes to the files. To save them to a CD again you will have to copy them to a fresh CD and burn that CD to save the files there.
Last year, Minecraft's web server crashed because too many people were trying to access it to register. So about a week after this, Notch re-uploaded the website, but didn't re-implement Minecraft Classic's saving. The save files are still there, but there's no way to access it as of now. Also, if you didn't pay for the full game, you can't save files to your computer, nor load them.
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Yes, you can. I saved a 140MB programme in my D drive and it works well. Go ahead and save.
make an account and then go into pause and save it i belive
If you have a D drive on your computer, then you can save files onto it. You might do so to have your data files on a particular drive, rather than having them all on the C drive. Your C drive is usually used for storing the actual programs. So it can be useful to keep your data on a separate disk. It is a good organisational strategy. It can make it easier to see how much data you have, as you know that anything that is on the D drive are data files. If there is a crash on the C drive, you could still have all your files safe on the D drive. So there are lots of good reasons for doing it.
Yes, you can potentially have unlimited. Whatever your computers memory can hold.