Good Morning,
In absolute number terms (current production), both Canon and Nikon have more lenses. However, when you step back a bit and consider that Pentax has in body stabilization and both C and N have lens based stabilization systems, their advantage in shear numbers drops substantially to just about an equal offering, essentially you are normalizing the each companies current set of offerings - apples to apples. Neither C or N offer any wide angle or ultra wide angle that are stabilized. With the in body stabilization any lens mounted is automatically stabilized. This is probably not a large differentiation, in that wide angle lenses there is no large need - however it helps, especially in low light if you're caught without a tripod.
On the wide angle side - landscapes and architecture, Pentax compares very well, the 12-24 and the 10-17 (fisheye), 16-45, 17-70, 18-55 across the zooms with the 15, 21, 24, 35 and 40 in the primes. They may be slightly slower in speed than what is available from C & N, but for landscape work is should not matter.
Third party lenses from both Sigma and Tamron enlarge the selection even more. Tokina use to produce lenses in the Pentax mount, however since Pentax and Tokina collaborated on a couple lenses (10-17 FE, 12-24, etc.), Tokina has dropped the K mount because Pentax does not hold at least 5% of the market. However, they also have older K mount lenses that may be of interest.
When you consider the older lens that are no longer in production, the selection get even wider. The current K mount started around 1975 to the present. Prior to that was the M42, of which Pentax offers a conversion mount for about $30.
One area that Pentax is light in is special applications like tilt shift lenses and teleconverters. Pentax had a TS lens years ago, and occasionally you see them available - they do not last long on the market. There are also several third part lens vendors that offer TS lenses in the K mount. Teleconverters is another that Pentax has nothing in current production now, however there is third party coverage - however there has been some indications that Pentax will bring something out in the future. Pentax had a 1.4 and 1.7 years ago that can still be found - again however its a hunt.
Moving into the future Tokina (also owned by Hoya) has a superb 11-16/2.8 (a design extension to the 12-24) that folks are hoping that Pentax will offer.
Probably the best reference on Pentax lenses, both old and current is at
http://kmp.bdimitrov.de/lenses/index.html
... so that is a quick overview - hope that it helps....