Second question first (it's easier to answer):
For Proof of Concepts, as a partner, or as an employee of Informatica, you only get timed licences. Meaning they are valid for four weeks (POC keys) or one year (partner or employee). Customers, however, usually receive keys which do not expire. Meaning you can use them as long as you want (and keep your server systems running).
Multiple license keys are quite normal in cases where you buy additional software licenses. For example, let's take a hypothetical customer named ACME Corp. At the start, ACME licenses PowerCenter on 8 CPU cores in production.
One year later they expand their license to 16 CPU cores. They now have the choice to get a completely new license from Informatica Shipping, or they can request a so-called "incremental license key"; in this second case they "add" the new license key to their existing license service, meaning that all of a sudden they may use 16 CPU cores for all their processes in production.
Another year later they purchase a license for IDQ, another 16 CPU cores. They now again have the choice to either ask for a completely "fresh" license (holding a license key for 16 CPU cores for each PowerCenter and IDQ) or ask for an "incremental license key" for the 16 CPU cores for IDQ.
The net effect of incremental license keys is that you don't have to unassign application services from an existing license service in the INFA domain, you can simply add your incremental license keys to the existing license and don't need to shut down any application service, not even for a microsecond.
On the other hand a handful of incremental license keys can make it pretty tough to for example set up a new box; you have to make sure that you set up the license service based on the license keys in the correct order, otherwise you may miss one particular add-on license key.
So its mostly a matter of taste (and of internal policies) whether to use one original license and n incremental license keys or whether to always request a new "fresh" license key whenever you purchase some new software or change your existing licenses.
Granted, there are more cases than I've described here, but this post is already pretty long, and more options might only distract you even more. ;-)