How many backups are kept in iCloud and for how long?
How Many/how long would appear to be a bit vague - possibly until you run out of space, but you can choose which backup to restore to from the ones on offer…

From How to restore an iCloud backup that’s older than the most recent one




  
  First, check out this article on how to restore an iCloud backup to an iPad (too much to précis here) —and yes, the process is the same on the iPhone or iPod Touch as it is on an iPad.
  When you reach step six, your iPhone will ask you to choose an iCloud backup to restore—either the most recent one, or an earlier backup. Each of your older iCloud backups will be stamped with dates and times, so pick a backup that’s dated just before the time you accidentally deleted that one contact from your address book. (Note: If you have multiple iPads and iPhones synced to the same iCloud account, make sure to pick the right backup for your particular device.)
  Once you tap the blue “Restore” button, the iCloud backup restoration process will begin—and remember, it could take several hours, depending on how many apps, photos, music, and other media you had loaded on your iPhone. For more details, be sure to read this entire iCloud backup how-to. same link as above