There is no real need to disable "extra" TTYs as under
systemd gettys are generated on demand: see
man systemd-getty-generator for details. Note that, by default, this automatic spawning is done for the VTs up to VT6 only (to mimic traditonal Linux systems).
As Lennart says in a blog post1:
In order to make things more efficient login prompts are now started on demand only. As you switch to the VTs the getty service is instantiated to firstname.lastname@example.org, email@example.com and so on. Since we don't have to unconditionally start the getty processes anymore this allows us to save a bit of resources, and makes start-up a bit faster.
If you do wish to configure a specific number of gettys, you can, just modify
logind.conf with the appropriate entry, in this example 3:
1. In fact the entire series of posts—currently numbering 18— systemd for Administrators, is well worth reading.