Pentax K-1 has a Pentax KAF2 lens mount and currently there are 163 native lenses available for this mount.
The latest released lenses that are compatible with the K-1 are the Carl Zeiss Makro-Planar T2/50, the Tamron AF 28-80mm f3.5-5.6 and the Tamron AF 28-75mm f2.8 XR LD.
K-1 also has a sensor based image stabilization system that is up to 5.0-stops effective which means that all of these lenses will be stabilized when used on the 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