The technically correct answer is "as many as you'd like."

The practical answer is "you may find that you don't like more than one." You'd be replacing "there's a laptop in my bag" with "I have to install updates separately on every computer" and "my client data files are not all in one place and need to be backed up from each location" (or you could solve that last one with "there's a hard drive in my bag").

Also, EFINs are location-specific; one EFIN covers one "e-files are transmitted from here" physical location. If you're e-filing from each satellite computer at its own location, you technically need an EFIN for each location. (This is also a potential issue with your laptop, if you efile with it from multiple locations.)