Zeb,
The key phrase is "at the same time". You can add and remove devices as much as you like, provided - at any one time - you stay within the limit.
For this purpose, each copy of the PC / Mac software counts towards the limit, as does each Kindle app installed on a mobile device.
If you are likely to exceed the limit, you can get round it by setting up multiple accounts, but then you wouldn't be able to share or sync content across all your devices.
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