most of the adverts i see for shares are around the 4, 5 and 6 mark.
rarely have i seen a 20 share. although they do exist.
a 20th share in a c150 for example, will obviously bring the fixed costs down to a figure almost anyone can afford.
if you do the maths re typical ppl flying hours, its plain to see that availability is not usually a problem. in my experience, most light aircraft seem to be parked up for days and even weeks at a time.