Pentax K-1 has a Pentax KAF2 lens mount and currently there are 141 native lenses available for this mount.
K-1 also has a sensor based image stabilization system which means that all of these lenses will be stabilized when used on K-1.
Since Pentax K-1 has a Full frame sensor, it has a focal length multiplier of 1× so lenses will provide their original focal length on K-1.
You may also also be interested in these:
Pentax K-1 Lenses with Image Stabilization
Pentax K-1 Lenses with Weather Sealings