Just in case you haven't seen this, I'll link it for your reading.
http://www.cichlid-forum.com/articles/peacocks.php
I would not just ask here, either, if I was seeking information on stocking a tank. I use as many resources as I can before making a decision on what and how many fish to put into a tank, as I think everyone should do.
The term peacock to me relates to a very generic term for a large variety of fish, ranging from 4" to as much as 9", and for the most part, are best kept in the standard mix of one male/3-4 females. Also, while known to be listed as mild and a community type cichlid tank fish, they can have aggressive tendancies.
Especially if not enough space to establish territories, not enough females, improper decor for hiding spots/security and a good diet.
I repeatedly see 55-75 gallon tanks recommended for them. While I personally think a 40 tall is not right for the long term adult size of a harem of the smaller peacocks, I have kept them in 40 breeders. But just the mating pair.
I could make a generic statement that I've kept peacocks in a 20 gal. But it was just for raising fry. Kept moving them out during grow out, to 30, to 40 to 55, and so on. With each upgrade moving fewer fish to the next size tank.
However, most aquarists, do not do this and I read your question as this is a long term set up. SO my personal answer would be none. I think a 55 would be better.
Stocking questions are probably some of the most heated issues. All fish are genetically different from one another and often have their distinct personalities. Like, some bettas will ignore shrimp and others won't. Fish keepers are a strange brood also. Every one's definition of proper care is different; in actual practice and what they proclaim they are doing.
Until we can design some pop up units for all fish, like those on Butterball Turkeys, that can warn us of over stocking, too small a tank and high ammonia, we just have to ask for advice, assimilate it, take a course of action and adjust as necessary.
Plan Do Analyze Change...repeat process