Install the package.
Busybox commands are symbolic links to
/usr/bin/busybox and thus take very little space. This is especially interesting for low-footprint systems.
The gettys are defined in the file
/etc/inittab, By default getty is started on ttys 1 through 4.
In order to enable/disable gettys, you just put this line in /etc/inittab.
tty2::respawn:/sbin/agetty -8 -s 38400 tty2 linux
Just replace tty2 with the tty, you want getty to start on. If you want init to ask you before starting the gettty, then replace respawn with askfirst.
See Gentoo wiki.