How many computers can I load Microsoft Office onto?

…if I buy the software and install it myself? If more than one, how long do I have to install it on another personal.

We’ve a desktop for our main computer that my son primarily uses and I have a laptop. My husband is starting his master's degree this fall, and I was thinking about getting him a laptop for his birthday in September.

So, if I have the ability to load the software onto up to 3 personal (as I've heard), is there a time limit built into software that won't grant me to use it again after a certain period of time?

Sorry if this is a “stupid question”. I don't really know that much about personal, and even less about software.

Thanks in advance.