This is the only thing I could find out:
donousa has a total of 120rooms to rent for tourists (and only in the summer), it has a total of 120 inhabitants (2001 census).
It’s distance from piraeus (athens port) is 120, so a bit closer than santorini, and santorini I remember took forever to reach by boat (about 7-9 hs), so it’s doubtful that they get much tourism from the mainland, probably people coming from other islands for the day…
Roughly, I’d make an estimate this way:
considering the only tourism they get is summer tourism, I’d say they work only for the months of may-september. 5 months, so 21 weeks.
Most people stay an average of 10 days (some 7 days, others 14, so I’m taking an average). So we’ve got 2.1 sets of tourists per room.
120rooms x 2.1=252.
Since each room is usually occupied by 2.5 people (on average)
I’d say there are a maximum of 756 tourists (that actually STAY there) per year, if we assume that the rooms will all be full all the time.
Personally I’d take that value and divided by half, if tourism was actually good a lot more greeks would move to donousa to make money off the tourists. They don’t… Which makes me think those 120 rooms are not always full, actually, they must be almost always empty… So make it: 300 tourists per year.
I can’t think of how many would come for just a few hours from the neighboring islands, it depends on how many boat trips they have per day, if they have one coming in the morning and one leaving in the evening and so forth.
But my educated guess is a total of 370 tourists per year total.