The DriverLog application was built to be driver and device independent.
This means that a tablet with DriverLog installed can be used in any truck in your fleet, or by any driver with a "Driver ID" in your account. So, you can have as many tablets as trucks, or as many as you have drivers, whichever is less. You can, of course, have more, but it is not necessary.
If you have 10 drivers and 15 trucks, you only need 10 tablets (one for each driver).
If you have 15 drivers and 10 trucks, then again, you only need 10 tablets (one for each truck).
If you have 20 drivers and 25 trucks, but only 10 drivers are ever out on the road at any given time, then you only need 10 tablets (one for each active driver).
The configuration is up to you. You just have to decide which is easier (or cheaper) for you and go with that option.