Hello, calculating DEM units depends on how many visits you will have on your environment. In general 1 DEM = 4 visits or 1 synthetic monitor visit. About visit definition there is good peace of documentation here:
That's not all information - this is just the description for a user session and how it ends from the product perspective. However, for DEM calculation, each started hour of user sessions gets counted. So if you have a 4.5 hour long user session, it will consume 5*0.25 DEM and not 1*0.25 DEM.
Here you find all information about how Dynatrace is calculating it's consumption (including DEM).