office 365 - how many machines can it be installed on?

A home office / single user business is looking to get office 365 business for the office suite and install it on his desktop and laptop and still have 3 available installs unused.

That's licensed per user.  If his son picks up the laptop and starts writing a homework assignment while he's on the desktop down the hall working in Excel, do all kinds of bells / whistles / alarms go off because 2 installs are being used at the same time? (the package is priced per user, so as much as you can install it on up to 5 devices, each user really can only use 1 machine at a time).
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BeGentleWithMe-INeedHelpAuthor Commented:
And along the same lines, office 365 is licensed per user?  I remember from the MCSE tests and questions about device vs. user CALs, they used the example of a machine in a factory break room.  100 people might use that machine over the day.  For that machine, a device CAL would be a better choice.

Does Office 365 have a similar user / device scheme?
Here's a blog post from MS themselves:

You have 5 installs available, for the purpose of using it almost anywhere you want/need to (it's mentioned for ProPlus version, but same applies to other editions. The install counter is there for a reason)
Basically though, it is still licensed per user. The user gets all the extra (OneDrive etc etc). If you are a family, and still decide to purchase only one license,  you will still have one OneDrive that has to be shared among the whole family. That's the reasoning behind it, they trust you'll purchase a license for every family member because of this.
BeGentleWithMe-INeedHelpAuthor Commented:
interesting info about activations / deactivations. But focuses on the 6th computer.  I'm thinking 2 computers in use at the same time.  And for a business install, not the home offering of $x for the whole family.
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It focuses on the 6th, because you are only at 2. It means, you don't need to worry.
Business also adds email/domain, so it doesn't really make sense to buy only one Business license, then only use 1 mailbox. But again, it IS possible, for up to 5 installs.
BeGentleWithMe-INeedHelpAuthor Commented:
possible (technically) but legal?
Bryant SchaperCommented:
No bells or alarms. While not really legal it is ok to install on multiple devices, Microsoft assumes a user may have a desktop and a laptop. Also I think business gives them the right to use at home activation key.  

But this is for the user, sense his son is not the user it is a violation but it will work

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