The backup for each batchfix contains copies of only the files that were changed by each fix, not every file. You do not need to keep them once you are sure that all the files changed in the batchfix are fine, and it's rare to have a genuine problem with the way that any of the batchfixes work. However, user error -- most often, accidentally running a batchfix on a directory that contains files that can be altered by the batchfix, but shouldn't be, such as running the 'disallow for random' fix on default replacements -- is NOT rare, so you should always keep your most recent round of backups at least until you've gotten into the game and made sure your Sims still have faces.
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