xTuple's license manager (in the commercial Editions of the software) counts only concurrent users connected to the database.  So while you'll want to set up accounts on the system for all of your named users, from a licensing perspective, we're only concerned with the ones that are connected at any given time.  It's generally a good idea to allow for one or two seats more than you think you need, to allow room for growth and unencumbered usage at peak times in your business.

The license manager also distinguishes between internal xTuple users connecting via the xTuple client or another xTuple GUI tool (CSV import, xTuple Connect, etc) ... and external connections to the database by other programs, including any web-based systems, database tools, and other custom applications you might have which need to connect to xTuple through the APIs.

For all Editions of xTuple besides the commercially licensed version of PostBooks, there is a five-user minimum purchase.  And for every internal xTuple user license you purchase, you receive an external license at no additional cost.  So if you buy a five-user license, that's five concurrent internal users connecting with xTuple software, and also the ability for five external connections as well.