The reason why a client would like to have a limited edition is that, if you happen to become famous in the future or if your image appears on TV and all the US want to have a copy in their living room, their "bet" on the quality of your work is paid back.
An art buyer can be seen as somebody thinking he has the taste to understand art before the greater market will. Art collectors see themselves as skilful investors. It was always like that. When the Colonna and the Pamphilj of this world built their "galleries" of contemporary masterpieces, the masterpieces were not necessarily priced as masterpieces. Only time gave them justice as fine collectors.
When a - say - Cardinal Scipione Borghese bought a statue or a painting, he was doing it not just for the pleasure of owning, but for the pleasure of "betting" on his own good taste and on his "nose" for art, and the pleasure of building a collection as a reflection of your own "nose" in art is probably not less than the pleasure of owning the art pieces.
Your art collection doesn't speak about your purse only. It also speaks about your taste and your intelligence.
If somebody buys a picture don't rule out that he does it in the same spirit of those great collectors of the past. He buys a work which he hopes one day to become a recognised work by a recognised artist. In buying your work, he's "betting on you" as an artist. There's also a number game here: you buy 100 works, and if 1 or 2 actually work out to be perceived as important works, that might compensate the expense on all the others.
So, to answer the original question, some people say that the right number is around 100 for a BIG image.
100 guarantees the buyer that, if you happen to become famous, his investment in your work will pay back. 100 doesn't really limit your likely income, but does give an incentive to a buyer: remember he might be buying not just for his personal use, but with an hope of a future larger reward.
Leaving an edition "unlimited" doesn't give any incentive in "betting on your work" to such kind of collector. You might sell to the merely aesthetically driven buyer, but not to the "old style" collector.
I say BIG image because if you happen to become famous and widely acknowledged you will honour your 100 limited edition for the large prints but you will print, in larger or unlimited editions, some smaller sizes of the same image. That is perfectly acceptable practice as far as I understand and will actually possibly increase the value of the big size photographs printed as limited edition.