This is determined solely by what you want to do with the soundcard. When multichannel
cards first started to appear, most people who needed more than a single stereo output
channel already had lots of rack-mounting effect units, and this is why various cards were
launched with a single stereo input, but eight outputs. These extra outputs can be used as
effect sends, so that you can carry on using your existing gear with a hard disk recording
system outboard effects can be added on a track-by-track basis, and an external
audio mixer used to reduce this to a single stereo signal.
However, since the recent onslaught of high-quality real-time plug-in effects, many
people are perfectly happy with a single stereo input, and a single stereo output, since
all the effects they need are available within the computer. Some cards have single in and
out sockets, but multiple hardware channels on the card itself (see Which Soundcard 6 for
more on this aspect).
You will need more than a single stereo input if you want to record multiple musicians
onto separate audio channels the minimum number of channels to record a live
drummer is probably four, and for a complete band eight is a more realistic minimum.
However, 8-in, 2-out cards dont seem to be made, so you are likely to need an 8-in,
8-out card to fulfil this function.