Per-server mode: You need a CAL for each concurrent connection to each server. I.E. With 25 client computers connected to 3 servers at all times, you would need 75 CALs.
Per-seat mode: You need a CAL for each client, regardless of the number of servers each connects to. I.E. With 25 client computers connected to 3 servers, you need 25 CALs.
In a single-server environment, it really doesn't matter which mode you use. Once you move beyond a single server, except for very specialized environments, it is generally more advantageous (and easier to track) to use Per-seat mode.
BTW, there is no requirement (either legal or from Microsoft) that you run the License Logging Service to track your licenses. As long as you know how many licenses you have at any given time, and know what licensing mode you want to use, you can turn off the License Logging Service. In my experience, unless you have a LARGE, complex licensing environment to track, using the License Logging Service is generally more trouble than it is worth.