Microsoft licensing can be complex but knowing where to find the actual licence terms is the first step in understanding the rules you need to comply with.
The licence terms which apply to the software you have purchased depend on how you purchased the software.
If you acquired the licence as a Full Packaged Product (FPP), which is essentially retail boxed product, then a document known as the Microsoft Software Licence Terms (MSLT), formerly known as the an End User Licence Agreement (EULA) will apply. Those licence terms appear as part of the installation process but in many cases they can also be viewed and downloaded from the following Microsoft website:
Each product, each Version of a product and in some cases each Edition will each have its own MSLT which contain its own product specific licence terms.
If you acquired the licence as an Original Equipment Manufacturer (OEM) licence, then the licence terms are likley to be different from the FPP licence terms but once again they can be accessed at the same URL as above.
If you have acquired the software licence through a Microsoft Volume Licence Agreement (which is typically the case for most organisations then the licence terms which apply to your usage are found in a document called the Product Use Rights (referred to as the PUR). The Product Use rights contains the licence terms for all Microsoft software available through Microsoft Volume Licensing. A new version of the Product Use Rights is released every 3 months as a general rule. That document, and archived copies of previous versions can be accessed from the following Microsoft URL:
You will also see another document available on this same URL called the Product List which contains other relevant licensing information.
So together with the actual Volume Licence Agreement contract itself, these two documents contain all the actual licence terms and conditions that your organisation will need to adhere to. But as I have pointed out above, these terms ONLY apply to software licenses acquired through a Microsoft Volume Licence Agreement. For FPP and OEM licenses, you will need to refer to the relevant MSLT or EULA document.
In answer to your question about Exchange Server:
Exchange Server, like many other Microsoft Server products (but not all) is licensed under the Server + CAL licensing model. In basic terms, a Server licence is required to install the Server software itself and then a Client Access Licence (CAL) is required for each User or Device that accesses the software (directly or indirectly).
Without going into too much detail, each device or each user which accesses the Exchange Server requires a CAL. You can assign each user a User CAL, or each device a Device CAL, or a combination of the two. A device which has a Device CAL assigned to it can be used to access the Exchange Server Software by any User. A user who has a User CAL assigned to them can access the Exchange Server software from any device.
From a licensing perspective it does not matter how many mailboxes the Exchange Server has. i.e. You can setup as many mailboxes as you like however any User or Device which is accessing those mailboxes will need a CAL.
As an example:
If you have a company with 5 users but need 100 mailboxes, then you will need 5 User CALs (or you can alternatively acquire CALs for the devices they use) as you only have 5 users who are accessing the Exchange Server sofwtare.
If you have a company with 5 mailboxes, but which are shared by 100 employees, then you will need 100 User CALs (or alternatively you could acquire Device CALs for the devices they are using) as you have 100 users who are accessing the Exchange Server software.
If you need any further clarification let me know. I hope that all helps!