Special issues with fisheye lenses
Fisheye lenses and adapters are very popular because they allow full-spherical panoramas to be constructed from just a few images -- as few as 2 or 3, more typically 6 or 7.
There are a few issues that are specific to fisheye lenses and adapters.
Nodal Point (Entrance Pupil)
Fisheye lenses and adapters do not have a single nodal point (entrance pupil) as most rectilinear lenses do. Instead, the nodal point of a fisheye lens depends on the angle from the lens axis, typically moving forward as the angle increases from 0 degrees to 90 degrees.
In theory, this means that it is impossible to get a perfectly stitched panorama from a fisheye lens.
In practice, stitching can be almost perfect if you calibrate your nodal point using the same angle that you usually use for stitching.
The autopano program often does not find control points that are good for fisheye stitching. It has a tendency to clump many control points in a small area, leaving large areas with no control points at all.
Manual control point placement is more reliable and often worth the trouble.
You do not need a lot of control points to get good stitching with fisheye lenses. If you have properly calibrated your a/b/c lens distortion parameters, then placing only 3-4 control points on each seam often yields good results.
Number of Images
In theory, 2 full-frame fisheye's are enough, but quality around the seams will probably suffer. (There are also legal issues here. One IPIX patent specifically targets this case.)
3 side-cropped fisheyes are enough, if you tip the camera so that sensor diagonal is vertical. This gives enough hfov to overlap around the equator. Zenith and nadir quality will still probably not be so good.
4 side-cropped fisheyes in portrait orientation is enough. 5 should be better but is not talked about much. (Maybe odd numbers make people uncomfortable.)
6 seems to be popular. Zenith and nadir are still liable to be soft.
Shooting handheld or on monopod, some people get good results with 6 around, tipped slightly up or down, plus a nadir or zenith to fill in the hole. That gives overlap everywhere.