You didn't mention your budget or if you're looking for a zoom or a prime. I used to own a Sigma 50-150 f/2.8 lens that produced excellent images. It's not a cheap lens, but it shouldn't be with the fixed f/2.8 and the quality.
Here's a link if you're interested: http://www.bhphotovideo.com/c/product/5 ... II_EX.html
I'm not familiar with Pentax lenses, but they may have one that would work too. Maybe someone that shoots with Pentax will offer some suggestions.
BTW, the lens won't have any affect on how many pics you can take. The lens will affect your shutter speed since shutter speed ties to ISO and aperture settings. The fastest zoom lenses are f/2.8 and will allow you the fastest shutter speeds unless you go to a prime (non-zoom lenses). Primes allow you to shoot using larger aperture settings, but they will reduce your DOF. The shallow DOF can be great when you're trying to separate your subject from your background, but it can also blur part of your subject if it's too shallow. Hope this helps.