A DS adds a few strategic elements to the game:
- Element of surprise / risk for attackers
- Possibility to defend strategic points and banker infra against superior enemy

I think these elements add to the game experience in a positive manner and thus we should have at least 1 DS.

Lets examine unlimited DS:
I played a few rounds with cloud pop-banking. His income was pretty extreme (with 144 mill pop) and enough to buy a new DS every 6 ticks. A full week of buying DS would result in 28 DS bought. Based on that experience:
- I don't expect any fam will be able to buy more than 35 DS in a round.
- Only highly optimized econ-families will have the resources to buy so many DS.
- These high econ fams will put the DS in banker systems, making their bankers virtually non-raid-able.

I think unlimited DS will give too much power to big fams as they will DS-up their banker core. 1 - 5 DS i.m.h.o keeps the strategic value without creating bankers that cannot be touched. Exact number for me depends on systems size.

i don't think that you figured in the increase cost of a new DS every time you buy one, as well as the fact the cost goes up as the family grows as well.
Add on that you could be under attack by another family, so now your resources are going to that, etc etc. 

I really can't see more than 10-12

So this in it's self would detur larger family's from farming the smaller ones.   Giving the atleast some sort of hope in the round