I tend to use Hugin to stitch my panoramas,
http://hugin.sourceforge.net
Hugin probably isn't the easiest panorama software to use but it gives good results.
The widest lens I've succesfully used is my sigma 10-20mm lens @ 10mm, 14 pics in portrait mode.
The main thing is to get a decent overlap between the images, try and keep the camera level (either using a tripod or a hotshoe spirit level, preferably both).
Also try and put the camera in manual exposure to make sure each image has a consistent exposure.
I'm not familiar with other panoramic software, but Hugin uses what it calls "control points" to align the images - I try and take my images so that there are good features which can be used to align, e.g. that power pylon in the distance, the corner of that field, or a window on a building for example. Try not to use sheep, clouds or people etc as these tend to move between images!
I find it also helps to use the camera in portrait mode as this gives a slightly "taller" panorama.